A little more anticipation to the fact that the zone is shrinking would be nice.
Also a trail behind the player would be cool as well imo.
I think having enemy variety would go a long way for this to feel more interesting as it's kind of brain dead play after 3 or 4 rounds in the sense that I only shoot to a lot of stuff that has little to no chance to kill me.
A good dopamine farm kind of game, I enjoyed it.