Very cool game! At first I thought it was infinite until I hit the boss (who died very quickly lol)
My one bit of feedback is that it would be nice if the "weapon" upgrade made the shot visually look stronger instead of just upping the damage. You could do this either by having chunkier/wider bullet sprites, or simply by upping the fire rate instead of damage. Consider increasing bullet speed a bit as you upgrade too -- it's not really much of an advantage but it will look/feel stronger to the player.