Really fantastic execution of this classic simple game jam genre. Procedural squash & stretch and VFX make super simple square graphics look great, the chromatic aberration is nice but not too much, and I really love the transition between rounds, that's an awesome touch. Awesome visual package.
The fact that there's actual terrain and it changes over time puts this above most arcade arena shooters, great simple improvement.
Multiple enemies and warning of where they're spawning makes this feel more high-quality as well.
The one thing I think this needs is a reason not to spam the fire button. Like most similar games, it's optimal just to keep spamming fire, which hurts the finger and isn't fun. Ammo, an overheat mechanic, or better yet alternate fires or a charge shot would help a lot. Also consider stuff like changing the player movement speed when firing or not, give the player a choice of how to interact instead of one single optimal way to play.
And while the changing terrain is great, alternate goals besides just killing enemies would be great too to vary the gameplay loop. For example, ammo, powerups, or health packs are a simple way to vary the play experience, but I'd recommend stronger design changes like every once in a while a specific position spawning the player needs to get to or else they lose health or can't fire or slow down, etc. There are lots of different design ideas to experiment with.
This game is extremely competent and impressive. Awesome job. I look forward to seeing what else you make.