Just a small thought on the collectibles... maybe if score numbers popped up while grabbing them it'd be a little more clear what they are? I wasn't sure if they were suppose to heal you, give you shield power back, or just score. Seeing little numbers popping up as you grab them would give a "score" feel for sure, if that's what you are going for!
As for the final boss arena, I get that it added challenge... it just felt more annoying than fun. I had plenty of challenge from the boss's own attacks. But then again, I'm not a bullet hell connoisseur really, so maybe I'm not the right judge. Either way, I managed to win on "normal" so the overall difficult is probably fine.