I focused exclusively on bullet upgrades and ended up with this screenshot (if it doesn't show on your side it's a spread of like seven massive bullets firing super fast. If there wasn't a 60 second time limit I'd be at 1000 by now)
Given that you made this game, you probably got to this point too, and I like how you didn't give the enemies more health or decrease the time faster, you just increased the explosions (spawn count) and let players have the fun because of the imbalance.
Also I only got to see the left of the screen because my window size is smaller than 1080p, but I still got to play the game because there were no important buttons on the right side of the screen, thankfully.
Edit: I officially fire in a circle all around me because of split shot