Very fun! I felt like enemies spawned far too quickly in the beginning. I was never able to fully recover from them. You might want to consider decreasing the spawn rate at the start and increasing the rate over time more slowly.
In the second round I played, I tried the "Wall Bounce Bullets" upgrade. Continuously running into the wall made it far too overpowered. I got a score of over 111,000 that round.
Good work!