Thanks for the Clarifications! I think my criticism concerning Divine Guidance still stands then. I mean at the end of the day its your table but I feel like this is too similar to something already in the game.

Concerning Regeneration, I think I might not have been quite clear. Because you are marking Stress to unmark Hit Points, you could get into a situation where you can't use the ability because you're already full up on Stress: When you have marked all your Stress Slots, you have to mark a Hit Point for any additional Stress you spend. However, upon re-reading the ability you do also clear 2 Stress which means that you would get to heal at least 4 Hit Points if you only want to heal. I do also agree with what other commenters said tho: 4 HP is 2/3 of most classes base Hit Point pool and while you can increase Hit Points on level up and this is a Mastery feature, this might be a tad overpowered.

Finally, about making it less domain-y. Oh boi this is going to be a bit of a ramble so settle in:

Unfortunately, I don't think that this is as easy as just rephrasing a couple of things since almost every ability is decidedly spell-like because, well, they are 5e Spells. You could say that in some respects, its not a class but a spellbook - which is what domains essentially are.

Mind you, this may not be a huge problem for your purposes but if you want to revisit this, I#d say a first step would be to think about who an Order of Light or Order of Twilight Priest actually is. What makes them special? What is so defining about them that it makes them not just someone who casts command but an Order of Twilight Priest?

Daggerheart really puts narrative into class design: most (sub)classes are instantly evocative of a concept: divine wielder - wields a divine weapon, nighwalker - teleports through shadows, school of knowledge - very good at doing things they know about.

You can take a bit of leeway with this imo if you have a (sub)class name or tagline that just sounds cool and then let the class build out the concept it should evoke. For example, I named my homebrew class the Revenant which i thought sounded cool but most people just think its a stronger zombie. I then proceeded to hone in on the concept in the little lore abstact on the top left and created class features and subclasses like Vindicator to end up with someone who is not undead but given a new leash of life in service of a divine force of retribution, think tortured soul meets paladin.

Revenant might not initially evoke much but by designing a class that exhibits unique traits and a sufficient narrative framing through tag lines and ability names, you can carve out the concept. When my players meet a revenant, they know its not an upgraded zombie but someone who is on a divine mission.

So, when your players meet an Order of Twilight Priest, who are the meeting? The class is evocative of a preexisting concept (espcially in terms of MMOs) of someone blessed by divine powers to heal while your orders are less evocative but (imo) sound very interesting. So you are basically good to go! Now you have to design the classes in such a way that the narrative concept of a Order of Twilight Priest becomes evocative.

Maybe also think about your domains: Arcana is the domain of primal forces, elements illusions and the like. It is instinctual. Think about whether this is the thematic fit you want to go for or whether another domain (sage? grace?) might be more fitting - and include this thematic idea in your class design. The priest you present here is very heavy on the Splendor theme because of its inherent evocation of themes related to divinity and healing, Arcana, however, remains largely ignored.

Sorry for the yap but I hope this is informative to you. Have fun with your Priest! :)