I‘m going to continue on this game after the jam. My plan(to implement your suggestion) is that you can choose up to 5 cards out of 10-20 random cards each round round/turn to build up a deck. When your turn/round is over you loose all cards that didn‘t played and can choose a new deck.
Another idea(but I need a lot of more monster sprites) would be that you building a fixed deck system(similar to Clash Royal). Each monster has its own fixed stats instead of randomly generated per card(as it is atm).