Ooo, thank you for the nice ideas :D
I've been messing around with the numbers and doing some playtesting, and yeah, the difficulty is definitely in finding that right balance between fun and unfair(and engaging/boring). I want to avoid it becoming a full MTG/Heartstone style tcg, but for it to still be an enjoyable change of pace between stretches of text. One minor change I'll probably make is booping the critical break damage up to about 5 - which so far has made the whole thing flow much nicer and allows for tougher fights with it still feeling more player-involved.
This is poteeentially heading into spoiler territory, but the idea of cards doing more specialised things was actually where I'm (hopefully) planning to take the combat in the future :O (characters would get character-specific cards that could do specialised things, like destroy a defence-slot regardless of its value etc. - with each character having a focus) - but that's still very much in the oven xD
I do like the strategy involved with holding onto cards, and I want to eventually try and create something that uses that, but that'll probably be when I have a clearer idea of how best to let it play to the player's advantage :O
Ah, also one more idea I have that I've played around with a bit is actually to have defence-slots hidden at the start of each turn - but have them able to be revealed by burning a low attack against them - although I might save this only for specific enemies...
But aa thank you again! I'm gonna try and get special cards working so I can really start making the fights a bit spicier :D