I tried this first in Firefox and experienced a massive amount of lag once enough enemies had spawned in, making it impossible to control (or lose). In Chrome it works much better, with just a bit of audio skipping at times.
Great aesthetics and a nice upgrade system! The player does feel extremely weak the first time around, as is typical for games with outside progression and the improvements to movement, attack speed and damage all feel immediately impactful. The extra attack is nice as well, but could perhaps stand to have a lower cooldown.
I played until I maxed out everything! At that point, since there's no end objective I'm aware of, I wasn't compelled to play further. Combat essentially boils down to flying in a circle while firing behind, there isn't a lot of room for more indepth strategy that I could find. It might be worth making the main attack active without the need for a button push, there's really no circumstance in which the player needs to 'save a shot' for any reason, especially as the fire rate increases, and it would free up the left mouse button for another type of attack.
Great job for such a limited time frame!