Let's start with the basics

Mythic+ Stat Priority

People tend to overthink secondary stats a lot when choosing gear. It's important to remember that item level is always king due to how strong main stat is. 9 times out of 10 you should be choosing your highest item level gear, regardless of what secondary stats it has. My rough rule for Leech/Avoidance/Sockets is that they're generally worth ~10 item levels. Below you'll find a breakdown of our stat priority for Mythic+.

Intellect
Crit
Haste
Versatility
Mastery
Main stat is incredibly strong this expansion and continues to be the dominant choice when gearing. This is unlikely to change for the rest of Dragonflight so make sure you're focusing on item level first!
Crit has gotten a huge boost in value since 10.1.5 because every build heavily utilizes Infusion of Light procs now. Don't worry about overcapping or DRing on crit, it won't realistically happen unless you have like 60% baselines crit which may be a bit overkill.
Haste is our second best secondary stat as you need a base level of Haste for this spec to play well due to the cooldowns on our core rotational abilities. While there is no specific haste breakpoints, higher haste levels means more mana strains which can be significant in raid. I generally like to stay somewhere around 30-35% so the spec plays well, but you don't burn through mana too fast or get overwhelmed with gcds.
Versatility is an great stat for Mythic+ as it increases both our damage and healing, AND provides us with damage reduction, which is vital for pushing keys. Use this as your filler secondary when you hit your desired Haste and Crit values.
If you just want raw healing throughput, Mastery is your best choice, but not by a significant margin. Remember it also has no effect on our damage, which in Mythic+ is a significant part of what we do. It's a much better stat now with the return of the Beacon of the Lightbringer talent, but in general is still the weakest M+ stat in most cases.
Raiding Stat Priority

The biggest difference between Mythic+ and Raiding stat priorities is that we place a lot less emphasis on damage and more on healing, which means stats like Mastery that provide a lot of healing but no damage will be prioritized much more heavily.

Intellect
Crit
Haste
Mastery
Versatility
Main stat is incredibly strong this expansion and continues to be the dominant choice when gearing. This is unlikely to change for the rest of Dragonflight so make sure you're focusing on item level first!
Crit has gotten a huge boost in value since 10.1.5 because every build heavily utilizes Infusion of Light procs now. Don't worry about overcapping or DRing on crit, it won't realistically happen unless you have like 60% baselines crit which may be a bit overkill.
Haste is our second best secondary stat as you need a base level of Haste for this spec to play well due to the cooldowns on our core rotational abilities. While there is no specific haste breakpoints, higher haste levels means more mana strains which can be significant in raid. I generally like to stay somewhere around 30-35% so the spec plays well, but you don't burn through mana too fast or get overwhelmed with gcds.
Mastery is taking a bit of a bump in the stat rankings this patch due to the return of the Beacon of the Lightbringer talent. Use this as your filler secondary when you hit your desired Haste and Crit values.
Versatility is a solid throughput stat in raid as it will give you more healing, damage and survivability. Generally speaking though it is our lowest hps secondary stat, just not by any significant margin so don't worry about actively avoiding it.
Fated Season Tier Set

For the fated season, our Season 2 Aberrus tier set has been resurrected with some tuning changes. Our 2 set got buffed by 20% while our 4 set got nerfed by 50% to offset the baseline buffs they gave Holy Prism in the last major patch:

2 Set: Holy Shock's critical strikes heal for 80% more and reduce the cooldown of Light's Hammer by 2 seconds, or Holy Prism by 1 second.

4 Set: Light's Hammer heals 100% more frequently and generates 1 Holy Power every 4 seconds. Holy Prism's healing is increased by 40% and it generates 1 Holy Power if cast on an enemy target, or 3 if cast on an ally.

This tier set will have a strong impact on our stat priority, talents and playstyle just like it did in Season 2. Holy Shock will once again be a powerful source of spot healing, allowing us to lean more heavily into Shield of the Righteous usage in Mythic+ and Light of Dawn in raid.

Playing around this tier set will simply mean pressing Light's Hammer / Holy Prism on cooldown and throwing out as many Holy Shocks as possible. Your default pick should always be Light's Hammer in raid as it will provide the most throughput, while in Mythic+ you should default to Holy Prism for the increased spot healing and on demand AoE burst.

Dragonflight Embellishments
Embellishments are a relatively small increase to your throughput, so don't stress too much over them. Below is a list of our best embellishments and which ones you should prioritize dependent on the content you're doing. For a chart of ALL embellishment values, check out QELive values and toggle between Raid or Dungeon on the top right of the header dependent on which list you're interested in seeing.
Best for: Mythic+ and Raid
Grants Vers for every ally (up to 4) within 30 yards.
Generally speaking, these will be your BIS embellishment for all content. At this point all embellishments have been nerfed to the point where none of them are very meaningful so don't expect a huge spike of throughput from this upgrade. This will simply providing your group with a small amount of damage and survivability.
Best for: Mythic+
Your damage abilities have a chance to proc Shadowflame damage.
This embellishment is extremely strong for pure dps, so it's of little surprise that it should be your go to embellishment for Mythic+. It can also be crafted on any piece of armor, making it very versatile slot-wise!
Best for: Raid
Your abilities have a chance to proc versatility for your allies.
This is your second best option in raid, but not by very much and also takes up a tier slot so it's likely not in your best interest to craft this until you have all your tier options so you can accomodate it.
Best for: Raid
Your abilities have a chance to proc secondary stats based on your gems.
While Lariat is a good late game embellishment, you should avoid it early on as it scales based on the number of sockets you have and most players will not have full socketed gear until a month or two into the tier.
Best for: Mythic+
Increases the duration of Dragon Isles potions by 50%.
Due to Potion of Chilled Clarity being nerfed, this is no longer our go to embellishment for raid. Instead, while not quite as strong as allied wristguards, you can use this in Mythic+ to increase your damage done with Potions of Shocking Discolusure.
Best for: Mythic+
Damaging a new enemy grants you Haste for 10 seconds.
This embellishment is actually very strong for Mythic+, but only in AoE situations and also takes up a tier slot which most players will not have the luxury to accommodate.
Best In Slot Trinkets
Below is an ordered list of the best trinkets you can get in the fated season of Dragonflight for raid and M+. As it is early in the tier, make sure to check back here for updates to trinket lists if tuning happens. For a chart of ALL trinket values, check out QELive values and toggle between Raid or Dungeon on the top right of the header dependent on which list you're interested in seeing.
Drops from: Vault (Broodkeeper Diurna)
Bonds you with an ally, granting you both versatility and healing
If you're running this trinket make sure to use this weakaura which tells you if you don't have an active partner. Broodkeeper's Promise gives you and an ally of your choice an absolutely massive amount of healing and a solid amount of vers. In raid throw this on a squishy melee dps, and in Mythic+ place it on your tank during trash and a melee dps during bosses with heavily AoE healing requirements.
Drops from: Vault (Kadros Icewrath)
Gain a secondary stat buff, increased by other allies wearing the trinket
As far as secondary stat trinkets go, this is the strongest iteration of one we've seen in a while, giving us a huge amount of baseline personal crit PLUS random other stat procs for everyone else wearing one in your party. Keep in mind that because of this, the more players in your group wearing this, the stronger it gets.
Drops from: Aberrus (Neltharion)
1 minute cooldown: Grant an ally an absorb shield. If broken, deal damage to an enemy
This probably won't be a super common trinket in lower keys or raid, but in high keys will likely see some use due to all the outplay potential an enormous shield off the GCD has.
Drops from: Amirdrassil (Fyrakk)
1.5 minute cooldown: Gain a massive shield and deal AoE damage
*This is a Mythic+ only trinket. Rageheart is one of the best trinket in the game for melee healers in M+ because it gives us a massive boost in survivability as well as damage and healing. The only downside to this trinket is you're going to have to fight off your tank friends to get your hands on one (or spend your fated tokens on it).
Drops from: Amirdrassil (Fyrakk)
Your spells have a chance to proc extra damage from your Critical Strikes
*This is a Mythic+ only trinket. This trinket is extremely powerful when it comes to raw damage in Mythic+, especially since we value crit so highly. The cap is very easy to reach so never worry about playing around it, just throw it on and let it do its thing!
Drops from: Aberrus (Neldris)
Gain a secondary stat buff, increased by other allies wearing the trinket
This is another great secondary stat trinket, providing us with a large amount of baseline personal crit PLUS other stats for everyone else wearing a different one in your party. Keep in mind that because of this, the more players in your group wearing this, the stronger it gets.
Drops from: Aberrus (Kazzara)
Your abilities have a chance to grant you crit and leech for 15 seconds
The stat budgets on this trinket are fantastic and provides us with personal survivability as well as a huge influx of our best secondary stat. It may not be as good as some of the previously mentioned trinkets but it's definitely a strong pickup.
Drops from: Aberrus (Neltharion)
Your AoE abilities have a chance to increase your Intellect
This trinket is performing much better now that it's working with our healing abilities. In general there's nothing very special about it though, it's just a pretty generic stat proc trinket so don't worry about playing around it.
Drops from: Aberrus (Rashok)
Your healing spells can restore mana and provide a HoT + vers to an ally
Not only will this provide a solid amount of mana return in both Mythic+ and raid, but it will also give a steady stream of hps and a stat buff to your dps friends. Seeing as it was nerfed however, it isn't exactly at the top of the list anymore.
Enchants and Consumables

Dragonflight reduced the amount of on use player enhancements by removing armor kits and replacing them with permanent leg enchants. Additionally, weapon oils were replaced with runes and flasks were replaced with phials. Before every dungeon or raid boss you'll need to make sure you have everything reapplied to your armor so I suggest getting a weakaura like this to remind you if any of your consumables are missing.

Item Enchants
Chance to increase your Intellect
Dreaming Devotion will provide your with the biggest boost to your throughput in raid, while Sophic Devotion should always be your enchant in Mythic+. Don't worry about tracking these, just enjoy the passive throughput boosts!
Waking Stats (Chest)
Increases your Intellect by 150
Waking Stats is simply a raw Intellect enchant, your other option is Reserve of Intellect, which gives you less intellect but increases your mana pool by 3%. While that may sound desirable, you will rarely use that extra 3% mana but you'll ALWAYS use the extra intellect from Waking Stats.
Increase your Haste by 82 / Versatility by 82
Which ring enchant you want just depends on your secondary stats, use this to increase whichever stat you feel you're lacking. That will generally be Haste / Mastery for raid, or Haste / Versatility for Mythic+.
Increase your Avoidance by 75 / Leech by 75
The general rule for these is to run avoidance in Mythic+ and leech in raid. In lower key levels leech will often outperform avoidance though, so you don't need to swap to avoidance until you get to a high enough key level that it feels necessary for damage mitigation (typically around +18s - 20s)
Increase your Avoidance by 150 / Leech by 150
Just like the cloak enchant, the general rule for these is to run avoidance in Mythic+ and leech in raid. In lower key levels leech will often outperform avoidance though, so you don't need to swap to avoidance until around +18s - 20s.
Increases your Intellect by 177 and Stamina by 133
Replacing Armor Kits, this is a permanent enchant crafted by tailors. Your other option is Temporal Spellthread which increases your mana by 4% instead of stamina. You should only ever run Temporal if a specific encounter is making you desperate for mana.
Weapon Runes & Gems
Increases your Crit by 310 for 2 hours
Weapon runes are replacing weapon oils from Shadowlands, and need to be applied every 2 hours. With Crits value being the highest its been since Legion relative to our other stats, you'll always want to run Buzzing Rune.
Increases Haste by 70 and Critical Strike by 33
There are a TON of gem options for us, just pick whichever one fills out your stats the best. Generally this will be Haste/Crit, which is either Crafty Ysemerald or Crafty Alexstraszite.
Phials & Food
Increases your Crit by 950 but periodically applies a DoT.
This should be your default Phial for throughput as it provides the most stats and the DoT can be mitigated by leveling alchemy and learning the Decayology talents in the Alchemical Theory tree. In higher keys you'll want to run Phial of Tepid Versatility instead.
Increases your Intellect by 76
This is always the most desirable food option as it provides main stat, but it does cost a little bit more then personal food.
Increases your Haste and Versatility by 67
If you or your group aren't placing a main stat feast, you should use this as your personal food since it gives both Haste and Versatility, our two favorite Mythic+ stats!
Potions
Increases your Intellect by 886 for 30 seconds
This should be your go to potion for either damage or healing in both Mythic+ and raid. It provides the largest boost to your throughput except on massive AoE pulls where Potion of Shocking Disclosure will pull ahead.
Drink to restore 48300 mana over 10 seconds
Nothing too special here, this is the typical "sleep" mana potion for Dragonflight. This will be the go to for regening mana this tier, as sadly the Chilled Clarity potion used in season 1 was significantly nerfed.
Instantly restores 1,500,000 health
As always, make sure you use your healing potions whenever you're in danger. You'll find these potions to be extremely strong self healing and off the GCD.
Gain invisibility for 18 seconds
If for any reason you need stealth, it's always a good idea to keep some of these in your bags. These come in even more handy if you don't have a rogue in your group.
Professions
Alchemy is the other major profession with a unique gameplay affecting component as you can talent into potion mastery, which grants you extra healing and movement speed whenever you use a potion. You also get increased Phial times so you'll save a lot of gold from that between Mythic+ and raid!
Being an Engineer in Dragonflight is not quite as required as it was in previous expansions for us Holy Paladins because we now have a Battle Rez. That said, it's one of the few professions that still has a unique gameplay affecting component to it via the Engineering "Tinkers". These tinkers include gaining invisibility, battle rez and much more.
That's it for character setup, now it's time to dive into the playstyle of Holy Paladin.