I'd probably pare down Domains to some ready prepped spells and a unique channel divinity manifestation. The Holy Orders would be the core that the subclasses would be built around.
You see, I'd rather them take the opposite tack and stay the course here.
Holy Order is akin to Pact Boon or Fighting Style. It defines your character's combat role, but it is the least flavourful decision point. It's integrated quite fluidly here with Divine Strike / Empowered Cantrips by merging those features into Blessed Strikes and pulling it out of the subclass options (something already extant in
Tasha's).
There's a limit of character concepts to expand into with Holy Order, much as there are for Pact Boon or Fighting Style. Subclass needs to be a broad enough concept that they can endlessly expand the choices with splatbooks. Domain works for that - Cleric is the gold standard of subclass expansion room, given that there are more Cleric Domains than ANY other Subclass, and often twice as many as some of the other classes. You take that away and build subclasses around Holy Order, and you're going to run out of party role archetypes quite quickly.
Scholar is the Expert Cleric, Thaumaturge is the Mage Cleric, and Protector is the Warrior Cleric. That's literally the 3 other class groups, and if you dip any deeper into them than this, then your Protector Cleric starts looking a lot more like a full-caster Paladin, and your Scholar Cleric looks a lot more like a Divine Bard, and your Thaumaturge Cleric looks a lot more like a Celestial-Pact Warlock. You start stepping on toes.
Maybe for a stripped-down version of D&D without dozens of subclasses, this would be fine. But if you want to sell books that give players options to express their characters' narratives in a more flavourful mechanic'd way, I think you'd do better keeping Holy Order as is and focus on the Domains.
4e tried the Holy Order as subclass in 2008. They scrapped it by 2010 and switched to Domains as subclass. The class was just too janky. See the 5e Mystic UA - it's trying to be a Psychic Warrior, a Psion, an Ardent, a Soulknife, and a Wu Jen all at once. And it unifies them by calling it the same class, but literally the only shared things are the d8 Hit Die, a couple proficiencies, and that they all use Power Points. Otherwise, they might as well be 4 classes rather than 4 subclasses of the same class. This was roundly criticised, since the warrior wasn't bulky enough to be a warrior, and the casters were TOO bulky to be balanced as castery as people wanted Psions to be, and the folks in the middle were kinda okay but they all just felt a bit tacked on.
Cleric has always operated in a middle ground between Fighter and Mage. But expressing that is tough, and been very swingy, where the Martial Clerics feel like either lesser Paladins or they're so good that they push Paladins into being Fighters with a few spells. Meanwhile, Pacifist or Laser Clerics emerged in 3e with the idea of Clerics being the Divine mirror to the Wizard, but that meant they lost their martial roots entirely and ceded it to the Paladin. Cleric can do all of these things by just dabbling in the other class groups.
I'd argue that's what the Leader role in 4e did. They dabbled in the other class groups while also providing healing and support. Cleric is right where it needs to be here.
I also think this is a great guide towards how they should resolve issues like the Death/Grave Cleric divide, the Undying/Undead Warlock divide, and the existence of the Hexblade entirely as an Otherworldly Patron despite Pact of the Blade being a viable Pact Boon. These concepts step on each others toes because they were iterations on the same concept with superior design coming later, or with slightly different roles in the party. Death Clerics want to be Warriors, but Grave Clerics want to be Casters. These "feel" like they should be the same domain, even if the types of characters who choose them are entirely different, because Death and Grave are closely related concepts. Maybe Death could be Destruction or something, but a better solution is just making it one more fleshed out domain that reflects all the roles of the Death Gods, and allow you to be either a Protector with a Greatscythe reaping the souls of your enemies or a Scholar/Thaumaturge putting the souls of the dead back to rest… or else summoning them to create an undead army.
Conversely, they can beef up Pact Boon and integrate it just a bit more so that you don't need to waste your Otherworldly Patron option on propping up your Pact Boon option, and can viably serve an Archfey but be given a blade by them, etc. We don't need to split off into Hexblade/Binder/2008Warlock sort of subclasses with Pact Boons, but they just need to be viable enough alternatives that every Patron can create good Warrior AND Caster Warlock servants via Pact of the Blade, Pact of the Tome, and Pact of the Chains (and Pact of the Talisman). Just not so much that Pact of the Tome feels like it should be a Wizard, and Pact of the Blade feels like it should be its own Arcane Gish Class.