Ey, I'm really thankful for such a detailed feedback! To be honest, balancing the stage was already pretty difficult and with the time constraints I couldn't think of more interesting bullet patterns like the boss ones, they had to be put together and with the shrink mechanic. It pains me so many people feel conflicted by the slowdown mechanic though, I put a lot of time into tweaking the speed changes and how it worked, and believe me it was slower on a previous build.
Like Zearoh said, maybe a tutorial would've helped, but in this case, I think making a focus button rather than when shooting would've worked better, but like other things, we didn't have time to implement and test them.
It is also the first time I make a stage of a danmaku / bullet hell like this, including researching... I would love to know more about these differences to make good ones, I'm mainly playing Touhou games, but I don't see much of a difference (on the first stages, the difficulty ramp quite high after stage 3 on those).
Anyway, I've read all your feedback and it is very much appretiated, I like critical feedback because it really helps understand what people see and think of the game, the game's problems and what needs revising or improving. It really helps me grow as a developer.
Thank you for taking your time writing this and playing the game!