Thanks for the super useful feedback and kind compliments, a lot of the points you mentioned are actually the things I am most annoyed about my game now, and are being worked on!
The super super difficult last stage was my cheap and nasty way of hiding the fact that my current ending (which definitely exists) is really sub-par, but luckily I have made changing most of the balancing elements fairly easy, so should just be a case of tweaking the data a bit!