Great idea with the looping arena! It adds a cool layer of strategy to the game that you don't get with most bullet hells. I found a bug where if you move left or right when at the very top/bottom, or up or down when on the very left/right (see the green rectangles), you can move out of the playable area. I don't know how you're detecting whether or not the player reached the edge, but if it's like something like an Area2D, making the are larger might help.
Other than that, I think a simple upgrade system where you can increase your base stats (damage, fire rate, move speed, etc) would improve the game a lot. I also agree with Lulullia that the player character should be more visible. Great work so far, keep it up!