Very interesting shooter concept! The colour palettes are very pretty and the game looks great - superb artstyle!
I would say the most important addition to this game would be a "reload timer" for shooting. Without it, the delay in shooting, as well as not being to spam the shoot button, feels frustrating. Once there is a visual cue for the reload time, it would take all frustration with that control away. You would then also be able to see the change the "Reload time" upgrade does.
The only other thing would be to experiment with the hitboxes for the enemies and the path of the bullets. If the enemies were not above me, I always needed to shoot quite far below them to hit. It sometimes felt like the hitbox was on the enemies' feet, instead of their bodies.
Overall, incredibly good-looking game! Well done.