Thank you so much for playing our game! A lot of what you suggest is things we planned to do but really just ran out of time to be able to fully implement well. For example in the boss fight we were going to plan different attacks like a spread shot with multiple bullets to add some variety and bullet hell-esq challenge, but ya know, time crunch haha. We would like to expand on this game in the future and add many of the features you mention in some capacity to make it a bit more polished.
As far as the health bar, I understand the confusion. I was going for a less conventional idea and didn't want to do the traditional "red something" for health, red hearts just feels lazy. Can totally see it being confused for mana because blue is almost always mana in games. Certainly food for thought.
Thanks again for playing the game and giving feedback! Cheers!