I like how which enemies are harmless change over time, keeping you on your toes. This has a good potential to bring about a risk/reward element where you have to choose between getting rid of the dangerous enemies vs trying to upgrade your bullet. To further improve the game, my suggestion would be finding ways to improve the skill ceiling, requiring players to figure out strategies for improving their score. One approach to doing this would be reduce the luck factor, such as ensuring that enemies do not spawn too close to you. Another approach would be to increase strategy by making it possible to lose the opportunity to shoot a mushroom that would upgrade your bullet (if you ignore that mushroom for a while).