Cool game with some interesting ideas going on. I particularly liked the way you made sure to encourage more tactical, sparing use of your weapon compared to holding down the fire button constantly, made for an interesting dynamic to the gameplay. I'll say that I found the starting shield size to be a bit too small, but the ability to upgrade this took the edge off this issue a bit, even if I'd still personally have liked it if it were a tad larger. Other than this, the game also had a really nice sense of progression to it, and the ability to upgrade stuff provided an interesting risk-reward dynamic given how it gave a reason to leave enemies on the screen for longer and amass a larger amount of money to become more powerful. While the game didn't vibe too well with me personally, I can really see the appeal of it and think that this feels extremely polished, interesting, and just well made in a lot of respects, great job with this one.
If you have the time, I'd also really appreciate it if you checked out my game.