I love this concept!
However, I did want to feel like I was making tradeoffs earlier, the board was so big on those first couple levels.
It feels like the equipping mechanic would feel better if the board was smaller and the powerups (especially the core ones) just a little smaller. I had to spend a lot of time fiddling between enemy waves. I think a smaller board would keep the concept but be tedious when you get a nice big item you want to equip. That way the focus could squarely be on making the interesting strategic decisions of wanting to carry a key or not.
The "charms" could be a 1x1 on a smaller board and I feel like I'd be doing less "busy work" rearranging things.
There is a very nice core idea here, though! Would love to see it evolved!