Great visual!
Gameplay-wise, the biggest improvement could be made by increasing the game pace. The more "ball sponge" enemies appear, the more boring it gets to just circle around the horde, slowly passing them hundreds of balls to get to the next wave. Maybe making some permanent upgrades could make the situation better, but this genre is time-consuming to find the right balance.
Another minor thing is character colliders. I guess, you've left them that big to make the z-ordering problem less noticeable, but as a drawback, it makes it feel a bit unfair in different situations.
Otherwise, great job making so much content in just 3 days! ¦3