Thanks for such detailed review. I agree that there should be some additional mechanic so that the player always has something to think about. Although, I don't like when there's too much to keep track of, and maybe with some tweaking the idea of the player entering pure surviving mode isn't bad for a bullet game. I like this idea because it introduces balancing between health and ammo, since you have this shared mana you have situations where in a pretty causal game the player is forced to change the strategy, maybe it's nothing new but I think it doesn't matter.
I think the theme is purely vibes, so maybe having an angel (final boss) as a demon doesn't make sense indeed.