Damn thanks, this is some really good feedback!
While I will probably not be able to apply all these changes soon, I will take this feedback with me when I'm working on the next update!!
I especially like your ideas of adding smaller rewards after each wave, adding a player stats window, and also the idea of increasing enemy variation by playing with size and enemy health.
Also thanks for the suggestions on the art style. The art is definitely the hardest part of making a video game for me, as I don't have much experience in that area.
Cheers!