Awesome! Looking at it this way, it's really more of an UX issue to teach the player the intended way to play. I didn't get the card merging at all.
I think another thing that I would've appreciated would be seeing the deck. Especially if I'm meant to strategize about using cards, it would be good to know what will generally be available.
An idea could also be to introduce a casting cost to cards. If you needed to kill a couple of enemies before being able to afford a card to play it would emphasize the strategic aspect.