Seems we both attempted to do card UIs that we've never done before and payed the price in terms of how much our vision was realized. Solidarity.
I do like the idea you're going for tho! Seems like it could be a really cool auto-battler! I knocked an enemy outside the map on the final level and couldn't beat it RIP.
I know you're aware of the issues but I'm just stating them so I can present my ideas on how to help with them:
One of the biggest problems currently is the lack of agency the player has on the actual gameplay. That is, I felt like I was just watching things happen. The one obvious one is to have more cards and ways to build out the deck. Another possible idea is to have an "active ability" that either initiates a special attack, gives a buff for a duration, or enhances played cards. My other idea is to replace the automatic card selection with making manual selections that go into a queue. Once in the queue, they will activate in the order that they were put into it. They cannot be reordered or removed once in the queue, so it takes planning to execute the cards you want in the order you want. I can't remember if potions activated immediately or during combat, but would be better to have them activate immediately I think.
Another issue is the UI tends to get in the way (especially for things below the character). The idea I have for this is to make the card area be "sectioned off." Either the bottom or side of the screen (both?). Meanwhile, have the game area its own contained viewing area to where the card UI does not overlap it and then center the character in that area.
Various smaller ideas: attack speed attribute (affects how quickly cards are used during combat). draw speed attribute (how quickly the bar fills to draw card). Draw card on enemy kill (independent of the draw speed bar) .
That's all I can think of at the moment. Looking forward to if you continue working on this idea!