Very fun game! I managed to get 7000 kills before my fingers got tired and the enemies became too many lol.
I have some ideas that I think could make it much better:
1: Enemy variety. As I played I felt the need to have some other type of enemy that actually made moving worth it, like an enemy that shoots from a distance and can kill your character. Right now there is no real incentive to move from the center of the screen as you can't take damage and the center gives you the best coverage for all directions enemies may come from.
2: Auto-fire. When you really think about it you realize there isn't a single moment where you wouldn't want to be shooting, having to click to fire actually makes it so someone that clicks slow can't take advantage of their fire-rate upgrades and someone who clicks fast will just get tired of clicking.