I guess it’s all about what you feel “overpowered” means. To me, that means something that breaks the game. Sorry, but I don’t accept that a one minute ability breaks the game of D&D 5e.
You just made a huge blanket statement.
If there's an ability that says "each enemy that starts their turn within 200 miles of you instantly dies during the next minute", that would be game breaking.
If you instead didn't mean that huge and provably wrong blanket statement, and instead meant "I don't think an ability that grants THP/this amount of THP every round for one minute is game-breaking", that's your own opinion, and I have said that from my experience, it has broken parts of the game (very specifically, it has broken certain encounters in the official adventure I am running with the Twilight Cleric in it; Icewind Dale: Rime of the Frostmaiden).
Then I suppose we define what “breaking the game” entails. I would argue that it means: making most or all encounters trivial and hence eliminating most or all of the fun at the table. Is that really what is happening? I mean, I see powerful feats and spells and abilities (looking at you, paladin auras) all the time. They do not impede the fun at our table. If anything, they encourage me to be more creative (read: not just throw MOAR enemies at them) in how I design encounters - something which I, and the players, find enjoyable.
Paladins are powerful, as are their auras, but their auras all come at a much later level than this feature, and Paladins are balanced more on being support-type martial characters, while Clerics are (typically) supposed to be support-type divine spellcasters. And, the auras are all much smaller ranges than this Channel Divinity until 18th level, and have smaller benefits than refreshing THP (yes, bonus to saves is good, as is immunity to frightened, but the bonus to the saves is dependent on the MADness of the paladin, and frightened is a very situational condition).
It’s also fun for us when a PC busts out a new powerful ability and it completely foils my baddies. I feign great disappointment and they get an upper hand in the fight or challenge and perhaps even an easy win. And we laugh and then I come up with more dastardly challenges for them after I cry myself to sleep later that night.
But that's the thing, powerful abilities are typically balanced by one of the following things:
- The ability being granted at higher levels (or only "coming online" at higher levels, like Creation Bards losing most of their restrictions on what they can magically summon at level 14).
- The ability being situational enough that the feature isn't broken (like letting Monks run on water and up walls).
- The cost of recharging the ability is expensive enough to balance out the feature (like Limited Wish recharging after 1d4 long rests).
- The ability requires a long enough activation that the feature isn't OP (literally the whole reason why Ritual spells exist).
Twilight Sanctuary is really good as soon as you get it (level 2), and only gets better as you get higher levels, so Balancing Factor #1 is thrown out the window. It also isn't situational at all, as THP is an extremely useful feature and also fairly rare (yes, the ending of the charmed/frightened condition is rare, but that's just some situational icing on the OP cake), so Balancing Factor #2 is also not applicable to Twilight Sanctuary. Next, it recharges on a Short Rest, which are very easy to take, and Clerics get even more CDs each short rest at level 6 and level 18, which gets rid of the last possible main balancing factor. Finally, Twilight Sanctuary only takes 1 action to cast, and it doesn't count as a spell, which means that it's not only very powerful, very rechargeable, and a very early-coming feature, but that it also is very easy to use.
By all means available to us (including comparing it to similar features, like Eldritch Cannon), Twilight Sanctuary is OP. The subclass would be powerful without this feature, but this just helps bump it into the OP tier.
It's OP. It really is.