The upgrading mechanic is really, really cool. I like that you put the upgrade list on the left so it's easy to tell what each upgrade does and how many you or the enemy have. The game itself is really fun, especially once you start getting lots of upgrades. Maxing out spread in particular is super satisfying.
I would make the tiny enemies bigger and make the background color darker, because it's really hard to see them. You should also add in a sound and visual effect for when you get hit. Since the screen is usually filled with bullets, it's very difficult to glance at your health bar to check how many you have.
Overall, fun game and a pretty creative mechanic. I managed to beat the boss too.