I really, really enjoyed this :)) It took me a minute to work out how to actually apply the controls, but I had a great time playing once I cracked it. The game was a liiitle bit easy overall, but it meant I got to progress right up until the end, which I'm not complaining about!
I would potentially suggest adding an arrow pointing to the side once a boss spawns, as it took me a long time to work out where the bosses actually were. Adding some more attack types to the bosses would be much appreciated too, like a horizontal laser or something that you have to dodge- though a full bossfight is a little wide-scope for a month long jam lol. Possibly also an explosion effect when the current boss attack goes off, right now the red light just disappears.
I also noticed that shooting while moving to the side caused the bullets to shoot on an angle, which I like a lot- but it felt a bit under-utilised. I'm not sure off the top of my head how to include it as a bigger mechanic, and I do appreciate the simplicity of the game as it currently is- but could possibly be a goal down the line if you keep working on the game.
I also really liked the upgrades, though I did very quickly become indestructible- but the attack cooldown was well-balanced, it prevented me from feeling too unbeatable.
Overall, I had a great time playing this one, and would love to see it grow! The gameplay was super fun :))