A top down horde shooter feels like a game every developer makes at some point, but I like how you incorporated the jams theme to add a bit more complexity to what is usually a very classic arcade style game.
First I want to talk about the art. Its all very functional and does a pretty good job explaining itself. That being said I would recommend keeping your UI in the same style as the games art. Mixing pixel art and standard art is usually a bad practice. I'm not as familiar with game maker but most engines have an easy way to swap a font out for something from https://www.dafont.com/ (Just make sure any fonts you grab are a free to use license).
As for the actual characters themselves I like how they look and move. And the little splash effect from bullets hitting things makes it feel a lot better when shooting. I also really like the little guys you used in the Itch thumbnail, but I wish they were in the game somewhere (Maybe the character select screen above each of the players options?).
Unfortunately the lack of audio does hinder the experience. Just adding some looping background music to give the game a bit more excitement could go a long way. If you're not somebody who likes to make music I recommend looking at Kevin MacLeod's library (https://incompetech.com/music/royalty-free/music.html). There is free to use music for just about any game you will ever make.
Last I want to go into the actual gameplay. I didn't run into any issues or bugs, and everything worked how I would expect. The player did feel a little fast for the small environment and would sometimes get stuck on walls, but once I knew to look out for that it wasn't a concern.
There was a large variety of buffs/nerfs that meant even with 6 options each round I would still get new options often. Balance felt fairly good for each of them and difficulty ramped up at a good pace.
Overall it's a fun little endless shooter with an interesting twist. I apologize if this review was a bit much, but I'm doing this to all the games from this jam as I think critiques help us grow