Thank you so much!
This is really helpful. Most of the deckbuilders on the top are done with much bigger budgets than ours (which is self-funded from savings), and many still go with pixel art. So I might follow this and see what happens.
Die in the Dungeon is actually a great example, as they have pixel art given here and key-art illustrations on Steam.
I think I will need to keep testing and see if CTR goes up... personally I know that what I find eye-catching rarely translate to the itch or steam game audience, so will just have to follow lead of others here.