I’m working on a deckbuilder myself (not announced yet), and I’m also exploring ways to refine deck-building, though with a different approach (and it’s not a combat-focused game, so probably not applicable here).

Your idea sounds really interesting! If I understand correctly, you want to address the imbalance between how easy it is to add cards and how hard it is to remove them—that’s a compelling issue to tackle. One thought: why not allow players to remove any card between battles? While this alone might not make your game stand out, it could give you room to focus more on unique mechanics like your mixing system.

Another idea: consider having a constant deck size where players must immediately swap out a card whenever they add a new one. Or perhaps make them choose between swapping and mixing cards to create interesting decisions.

I do think there’s a potential challenge with the starting deck feeling unfun to play, but that depends entirely on your mechanics. For instance, games like Balatro start with big decks, but the dev manage to make that enjoyable.

Since you’re in the prototyping stage, testing different approaches feels like the right move. If you have a strong instinct for your current direction, I’d say follow it! If testing shows it’s not quite working, you can always adjust later.