Played it on easy, it was a piece of cake. The played it on hard, was also pretty easy. Overall, when you get your first upgrades, you start gradually balling out of control.
Also, surprised, that you cannot shoot diagonally, like in Pomao. And the fact that the character's speed affects the bullet speed, while realistic, makes aiming more difficult. Upgrading the speed only makes it worse, cause bullets become more affected, also it becomes harder to line up shots, cause you tend to "overshoot" due to moving faster. Unfortunately, not upgrading the speed is not really an option, cause you have to backtrack a lot. I'd probably lower the speed and add running.
Anyway, I think this game has potential if you add more enemies and upgrades and tweak the difficulty so that, I dunno, it increases the further the character is from starting room. Also, a minimap and ability to teleport to rooms(like in Enter the Gungeon) would be really useful.