I like your concept and had some fun playing it. I like the mechanic of spinning the mouse to destroy the demon soul. The bosses are aggressive, which I actually like.
I think the control can be improved. If you jump without moving left or right you dont jump as high, personally Im not a fan of that. I also think you should hold to activate soul siphon and release to deactivate instead of toggling.
For boss design. One thing I noticed is that their hitboxes are not clearly indicated, for example, the first boss I got confused why Im not damaging him to only realize that you have to hit him in the head. Bosses health are also really high, which lead to long repetetive fight in my opinion.