The transparency aspect is super cool, such a neat mechanic.

Suggestion: If the cards that are able to be played on top of other cards are limited in their usage (like with the example you showed with the battle turtle + knife, can you only use 1 knife card on that turtle? or can you stack up multiple?), you could redesign the layout to show like a "character" and then their "item slots".

I've drawn up a super professional high-quality print-ready ms-paint mockup to better illustrate what I mean:

Not sure if this works with the type of gameplay you're thinking of, but could be a solution. There's other layouts you could explore of course if you want the item cards to be more "full" and have less negative space.