I started playing without reading the comments and was shooting non stop. I only realized the bullets were looping after dying like 4 times.
I think the idea is great. It would be helpful to give some feedback that the bullet is looping, like they enter a tiny blue portal to the right and the come out from a tiny blue portal from the left. This would help teach the mechanics of the game. Also, a bigger shooting cooldown would help prevent players from making huge mistakes when learning the game, as shooting non stop is usually the strategy in these kind of games that doesn't have ammo, so many will have this natural instinct.
This set of mechanics generates some interesting strategies like creating a 'vertical' barrier of bullets on each side, or even a rectangle of bullets looping around the center area to protect you. But some enemies disrupt this strategies by shooting or bouncing. Really cool concept
There is one thing that bothered me though and maybe It could be improved. I thought the player character was too slippery to control, it maintains momentum quite a lot. Adding to that the after image trail effects, it becomes hard to control the player while not looking directly at them (which is what usually happens in games with a lot of bullets like this). I would test 'increasing the friction' and resolving the readability of the player position. Maybe add a 'cross like' sprite overlay to the player, so you can tell the x,y position with more precision with peripheral vision.
Also the game ramps up really fast on difficulty, but maybe it's on me haha.
Congrats! o/