Some QoL ideas:
Upgrade that allows lower tiers of an equipped card to recharge at lower speed.
Card UI shows lower tier of that card recharge progress (since the cards already have a gray overlay to indicate charge, how about having one/two rows of pixels have a negative/high contrast overlay to show that?)
Be able to instantly place a high tier card, skipping the whole "place tier1, swap, place tier 2, swap, place tier 3", even at 0.5x, some enemies are too fast to react.
As you can see, I despise how much of a chore the whole card swap is, I mean, it's cool and all, how many units there are, but if it could be a little more streamlined, it would feel so much better.
Keep up the good work, have a nice day.