I think the reserve needs to be closer to the game scene, at the bottom of the screen maybe?
Love the keybing shortcut idea though, I use that a lot in slay the spire and it feels good to use. I might run out of keys to bind though, as you can get up to 15 units. Would not mind a cap on the player inventory to 8 or 9, to force the player to capture specific pieces first or discard unwanted ones.
I just really like thinking about this game there is a lot of ways to improve everything across the board! But the base is solid, easy to expand and i think already kinda fun!