So it's a lot like Vampire Survivor, which I don't mean as a dis.
The progression is a bit wonky: sometimes you get a ton of updates almost instantly, sometimes you get updates that take too long. I would tie the updates to time rather than money. Particularly when you have so much capital in the end game.
Also, you need upgrades that have weird, and unforeseen, synergies. Bullet shield but also heat seeking bullets combine to make a new kind of bullet shield. etc. The fact that there is a linear progression of guns is a bit more boring than needed. I want to be able to get the kind of gun I like, or one that combines with other upgrades to be super powerful. The upgrades themselves should be somewhat randomized rather than a linear progression. To optimize the fun, if someone gets a lucky break with the upgrades their score should be much higher, slot machine reward, rather than someone who got a bad hand.
I'd also like to see "auto aim" as an option because I'm just lazy and want to shoot at the right target without thought. Also, the missile guys seem to not aim at you properly? That made them a bit easier.
I died once and it seems like that had zero consequences. Not sure how stiff you want the penalty to be but it should cost *something* I'd imagine.
Also, the goal of blowing up those two AC units was unclear so I just sat there getting upgrades for about 10 minutes just to see what would happen. If I had known it was about how fast you can kill the AC units I would have worked a bit faster.
As we've seen from Vampire Survivor, this game has the potential to be a smash hit, but now it's already got a lot of competition in the space. I personally would like to see one of these where you can build up the terrain and make walls and bunkers and stuff. Like the waves only come after you build up your defenses a bit. Maybe place some turrets, etc.
Anyway, nice job.