Pretty fun, Rockets + Damage buff + Shot Speed are OP. I really like Geiger-esque enemies.
Felt like the game would be too easy with regenerating HP and a slowduwn, but I was VERY wrong - later levels are pretty tough. Though main enemies were much tougher than the bosses. Again, rockets are OP, so I kinda breezed through, dying only on last levels a couple of times.
Levels felt like they repeat, no idea whether they do and only the HP of the enemies was changing (I don't think TTK for bosses changed).
But I really like your upgrade system, though it feels like some combinations or upgrades are complete trash (like the multishot). Overall feels like a good proof of concept, really cohesive, but needs more content.