I played the game in version 1.5.1.10, so there wasn't any save system (at least, I saw nothing at the usual Unity save directory)
I got that weird visual bug after removing a card from my deck for the rusted sword curio. The other UI bugs, you certainly already saw them during the streams.
I really thing adding a combo value visualisation would be a great thing, it would make balancing the deck to have the roughly the same amount of 0, 1, and later 2 and 3 combo cards. It is basically like planning the mana curve of a magic deck.
I thought the hiding of the enemy intent could be by design. Having played a good amount of Runic Done runs in STS, it adds a good amount of randomness. I don't know what to think of this, currently the enemies' patterns seems to be quite predictable, so I don't see the reason to hide them if the player could just find them on a wiki. Showing an action list could be a good compromise. May be you have ideas to build upon that particularity, in that case I wish you the best.
I beat the game, yes (see my previous screenshot against the final boss).
I'm at the discord server limit, but be sure I will be looking at the game in the future.
I encountered the mimic twice in early run. The first time I ran away. The second time I beat it, after a very difficult fight that cost me my revive relic.
I didn't saw any other secret bosses, at least yet.