Really neat little game! The aesthetics are very crisp in a way I liked a lot. It's also a neat concept: a ticking timer is also a resource to be spent.
I do have some criticisms of the design however:
My main complaint is that I think the balancing of the costs of everything is way off, in that it takes way too many rounds to get enough money to buy the next upgrade. I think this would be true for any game, but I think it's doubly true in a jam submission, where you really want things to be big and fast and punchy to show as much of the game as you can as fast as you can.
Ideally I want to be buying an upgrade every single round - multiple upgrades if I perform really well. In practice, I was probably having to play like 5 or 6 rounds minimum to afford the next upgrade. Which ends up meaning you're doing the same thing over and over again without introducing anything new. I really wanted to be able try the shotgun and the sniper to see more of what the game had to offer but I unfortunately don't have the patience, which is a shame because I do think the game is really great!
For next steps, I think the game was at it's most fun when I was able to line up efficient shots once I had pierce, so I think to elevate the game to the next level I'd love to see more opportunities for skill expression. Maybe you can move freely instead of being fixed in place, meaning you can use skill to line up good pierce shots. Or maybe if you had a movement system then mastering that could let you kill guys faster.
Overall, really solid entry! You should be proud!