Played this for longer than I'd like to admit, the upgrades are quite rewarding :)! I think adding some variety to the enemies' behaviour would be great, perhaps an enemy that shoots or dashes at the player… I find fiddling with the physics of how an enemy moves can add a lot more character to them!
Good job on the game.
PS: If you tick the "Windows" checkbox in the game's uploads, it will make it possible to download the game through the Itch app!