Got done with the demo, good stuff! I like the screen spin mechanic, I've been curious what a bullet hell shmup would be like with 'Gyrus' or 'Tempest' spin mechanics. I think you pulled it off quite well. With more arenas/bosses and some light story & I think you'll have quite a fine full game. Looking forward to it if you continue development.
I was pretty satisfied with the challenge, but as stated in some other comments, don't be afraid to bump the difficulty up just a bit for later bosses.
My one real gripe is the mouse - seems kind of unnecessary gameplay wise and not great for use with control pads or arcade sticks (which can offer finer control than keyboard for tight bullet dodging.) Maybe have an limited lock-on feature that encourages players to spin the screen towards the target?
Also, the Linux build runs fine; was able to get through the whole game no problem.