Nicely done. Feels pretty good. Congrats on finishing your game!
You really nailed your aesthetic one this one. It feels good and it's satisfying to look at. The gunplay and ramping difficulty felt appropriate (if a little slow - more below). And the hit feedback and player feedback are fantastic. There's clearly a lot of polish and shine and effort put into this and it shows. Well done!
I think you could have made the game significantly shorter though. The levels take a long time relative to what is being introduced. The first stage is the same enemy, in various waves, over and over and over. It would feel a lot snappier to only have one battle (with multiple waves) or one wave per battle. I lost during the final(?) boss and really wanted to finish the game, but the prospect of replaying it felt like a grind. But that could have also been helped by the rewards.
Your premise of "if you get a buff, so do they" is super compelling! I was a little disheartened to see it wasn't symmetrical, but that's understandable. That'd be a nightmare to balance, and then what sort of upgrades do you give enemies that can't shoot that would also feel good for the player? But I think the bigger thing was giving the player the choice to pass on upgrades overall. That's effectively giving the player the option to not engage with your core hook. If I opt out of an upgrade, then I'm playing a generic shooter. Giving a choice between two would force engagement, make for more salient escalation in difficulty and player power! And because later stages would still upgrade different enemy types, no single enemy would never become too obnoxious.
And on the topic of rewards.... why don't bosses give rewards?? Those are big moments with big fights and new mechanics, but the reward is.... just more stages. No choices, no power ups. Maybe this came down to a scope issue, but I think the game might have been okay without the bosses then. Right now, they feel like unknowable damage sponges. No health bars or sense of progress on them didn't really help either.
And I say all this because I REALLY liked this game. I can tell a lot of care and skill went into making this. You should be super proud of this. It's a cool little game with some very fun design choices. Please keep making games. This is cool.