Thank you for your review!
Your suggestions are good, but I actually made this game just for the jam and have no plans to develop it further, but maybe someday.
I already thought that some levels are very difficult, the solution for level 8: You have to go to the left and then go on for a long time, just before the finish there are more spikes. The idea was that the player might not realize this until he cleared the long spiked platform and then hit the long platform at the bottom.