Nice, my kinda game! Becomes rather easy if you manage to survive the early game to snag up the right upgrades. I intentionally died here as my 50+ max hp was way too much for the enemies to do anything to and it didn't get any harder anymore at all.
So basically my strategy was to never get speed, prioritize bullet count & shot speed and max hp, piercing & damage are pretty good too. And for the enemy, always pick count if available, don't give them more hp so you can instantly kill them. And always be shooting! That's why I'm of the design school that if you have a button for something like shooting, there should be a reason to sometimes not do it, ie. reloads, slower movement etc. Might as well be automatic otherwise.
It was really confusing at first that why suddenly I couldn't kill the triangle ships like I could earlier but few loops in I realized that the target changes every time. And the targets were balanced quite weirdly. The square one ends before I get to see more than few enemies, the triangle ones were medium but the only threat was the circle ones and if they decided to be too sparse and you have an armada of triangle ships harassing and shooting at you. But even those with proper setup weren't much of an issue.
Liked the pixel art! I'd make the background a bit less bright so it wouldn't steal the show that much and it could be easier to focus on enemies & their shots with your side eye. And the background tilting looked cool but was kinda nauseating and disorientating.
The audio stuff wasn't bad either but I think it was the weakest side of the whole game. The music was nice but it didn't really fit the mood. Something more hype and hectic would have been a better fit to match the action. The sound effects were kinda sparse too so they kinda got boring too. And I think the music volume was a bit too low in comparison.
One more irritating thing was the constant need to switch to mouse for the purchases between. Wasn't that bad since you could also shoot with mouse but that kinda forced the control scheme to that even if you gave some alternatives.
Anyways, good job! 👍