I think Conclave seems really weak and not interesting at all, but maybe I'm missing something.
I think Conclave:Festival::Imp:Laboratory (except for missing +1 buy), approximately. Among the 96 $5'ers in the Qvist rankings, Laboratory is 23 and Festival is 58, so maybe this'll be a middling $4-coster?
To serve as the sole village of an engine, it needs multiple different cards that draw. The fact that it doesn't draw a card by itself restricts its use quite a bit. On the other hand, it provides economy while you build up; maybe that compensates for it being a... shall we say wonky village?
Monastery will obviously be really strong. It only costs 2; It is essentially non-terminal, and it can trash coppers that are IN PLAY. Obviously, this is not quite as good as chapel, but it can really do some work I think.
This is not obvious to me.
Trashing is most valuable early. Number of cards gained per turn is low early and high mid-to-late game. The effect of getting to play the copper you trash is roughly comparable to the +$1 from Forager, another non-terminal trasher—unless you can play and then trash multiple Coppers.
I think the best case is something like "play Ironworks to gain a card, buy a card for $3 and trash two Coppers". That's a pretty strong turn
12 to 15 3 or 4 turn, but it's also somewhat unlikely. More likely is something like Monastery, Estate, 3xCopper. Here you buy a card for $3 and trash an Estate; a Salvager or Moneylender would've let you hit $5 (but they also cost more).
I think it'll be a
pretty decent trasher; probably somewhere between the better single-card trashers and the weaker multi-card trashers, which is... just exactly among the top ~25% of trashers? Maybe?