Just tried my hand on Normal Mode and wanted to give some feedback - the initial difficulty curve was really daunting at first because the flight patterns from those enemy ships are a bit unpredictable, but as soon as the power ups came to play, it became easier as opposed to harder, so if there's a way to balance the difficulty curve either by making the initial flight patterns slower, and then speeding it up or having it do more crazy hairpin turns, it'll help.
I must say the boss fight was really good though, it really brought the controls back together into a nice little package where you'd have to figure out how to beat that monstrosity :)
Oh also, for some reason, the music and sound effects stop playing once I've reached the core of the boss fight and/or after my health has depleted more than 3/4 of the way. Not sure why.
Loving the game's aesthetics and music too, keep it up!