It was really fun! It completely felt like a good ol' arcade game- the music in particular was on-point in that regard. Except for a slight lack of polish (which given you made it in only 3 days isn't surprising: it was impressive how much you've done in such a short time) it wouldn't have felt out of place in an arcade...
I'm sure if you'd had a little more time you would've done some of these, but the only few tweaks / polishes I would've made myself are-
1. Turn the voice audio up (it seemed very quiet compared to the guns/sound effects)
2. Just add a little check so you can't get the same upgrade option twice (or thrice) in a single wave
3. Balance the weapons a little more... grenades were a little OP, everything else seemed a little weak... I think a cheap / lazy way (aka the way I would've done it lmao) is to make the grenade launcher a bit worse too (longer reload time, perhaps?) and rather than balance the weapons against the enemy strength, I'd've just slapped a 'difficulty' slider on the main menu that just set the enemy health / spawn rates / damage lol
All in all it was a load of fun! And if this was the sort of game that I was into (I'm a total nerd, so overly in-depth, niche simulation games are more my thing lol), I can totally see myself spending waaay too long playing lol. GG! :)