Actually I think the Slay the Spire analogy is perfect. It would take too long for a single floor challenge of course, but aside from that imagine a little extra challenge in early act 3 where you get to draft a new deck from a limited selection of more powerful cards, run through a short gauntlet of specially balanced fights with it (maybe drafting a few new cards each fight), and then get a reward (perhaps getting to keep 2 or 3 of the cards you drafted). I think something like that would work pretty well within Slay the Spire’s existing systems.
As I’ve said multiple times the entire quest would be balanced around itself. Probably the items would not be outright OP and wacky but you’re also definitely not going to have to basically play a second sewers.