Thanks for the feedback, yeah to be fair I generally don't play nor make bullet hell style games, so this was kind of a first. Honestly I agree with all the points, I wanted to do stuff like more custom upgrades, some more enemies and a boss or two, but I just didn't have the time to do it, with the little time I had to actually develop this game. Though I would say that the difficulty slider idea could have been added if I just played more games of this type and knew that that is a thing that's done.
In terms of the update written on the bottom of the page, that was something that I fixed in the 24 hour period where we could fix some stuff, technically wasn't a game breaking bug, but it was too difficult so the damage enemies deal and the accuracy of tracking was decreased, that's why it feels more or less good right now.