I got carried away and had a lot of work to do, so I couldn't really play this game until now. Damn...I should've played this earlier xD
Gonna start with some problems or improvements first, the game doesn't have fullscreen. Sad :( Not game-breaking though so it's fine. Also, the settings lack other options. And during the levels, the movement is a little bit slow for me (Although this may be what you wanted, and if so I actually understand why as well). And another minor ish thing is that the background of the levels could be a bit darker or sort of muted, so it can have better contrast and you can see the players and enemies easier.
Despite those stuff though, this game is seriously really well done, and much better than I was even expecting. Despite the fact that it also looks like a mobile game (especially with the massive menu buttons, although not a bad thing), it feels really polished and well made, especially with you and your brother making everything. And yes, the visuals and artstyle are also pretty simplistic but really nice to look at, and the visual effects (especially the explosions and stuff) are really nice and well made. The game itself is hard. Really hard...But pretty fun as well, especially the random shooting mechanics, and personally the enemy spawning weapon makes the game a lot more challenging but also really interesting and fun.
Not gonna make this too long again, but overall this is a really well made game, and I genuinely quite like it. Keep up the work!