Using time to buy upgrades is a really cool feature! I'm honestly surprised I haven't seen this before. Not going to delve too much into suggestions here as stuff like balancing issues (bullet count being the superior upgrade), lack of enemy variety, and the boss bullets being the same colour as the player's have already been mentioned, but I will say adding a volume limiter to the audio should take priority over anything else. The gameplay's fun and all, but the crackling of twenty ships dying at the same time takes away from the experience. Anyway, you got a good foundation going here, so best of luck if you're continuing the project after the jam. Keep it up!