Interesting idea! just like everyone mentioned. I have never made this kind of game, but I have some suggestions as a player.
time is crucial, so bullet instantly hit will be great. rather than aim from one spot, shoot straight, and let the player move between vertical tiles seems more convenient. It's a hectic game so consider auto-reloading the gun.
the money feels not really needed and the difficulty needs some rebalancing, but that is common in Game Jam.
Overall it's a nice work. you have a really good idea.