Cool entry!
Gameplay wise, I think the game is pretty solid, and it could be a very strong foundation to build something that's really complex and offers lots of choices. I think your powerup system allows for very cool level design, since you could have a lot more optional routes that depend on these powerups. Being that the jam is about speedrunning, I think this is a very good thing, since speedrunning often requires players to cut corners and use different things at optimal times. I think you could make the powerups be key-accessible, with the 1-4 keys, for example. That could make it so that you don't have to choose between shooting and selecting a powerup. I also agree that it could be a good idea to have some sort of trail effect for your bullets, since it could be a bit more clear where you're shooting. I think the method you've chosen for the fire attack damage could use some work (I think you gave your particles colliders?), since it often feels like you can be hit by particles that are almost invisible and you feel like you're taking hits that you shouldn't be taking.
Themewise, I think the connection could've been a bit stronger. The end of the level doesn't really feel like I'm cooling down after overheating. Maybe this could be achieved through the presentation, like maybe you have some sizzling sounds, a water splash, or some water drop particles or vfx when you've finished a level, stuff like that.
In terms of aesthetics, I think the game looks very pretty and coherent. I specially like the spider's faces before they're about to attack. Very nicely done!
I think the music choices conflict a bit with one another. The very laid back music you have for the start menu feels like it can clash a bit with the very upbeat and happy music you've chosen for the levels. They also have very different vibes, with one being sort of like a lo-fi "beats to study to" sort of thing, while the other feels like chip tune-y video game music. Of course this is all completely subjective, but my suggestion would be to change the start menu music with one that feels more chip tune-y while preserving the laid back vibe. The sound design works well enough and compliments the gameplay nicely, since actions feel responsive and contextually appropriate.
Very cool game, I think you have a very solid entry on your hands! Keep up the good work!