Thanks a lot for the detailed comment!
You're correct that if you just spam click you can go slow and it's quite easy. I think if I expanded the game I would do some things to force you to go faster and use larger planets (which then require you to slow yourself down by putting planets behind you)
There's actually 0 checkpoints so I'm glad you made it! The ending especially was intended to be experienced just the way you described it, tensing up at the end of a long level when you die in a single hit. Love to hear that I succeeded in offering that experience.
The very last level is actually not that hard (Unless you go really fast) but the 2nd to last one I would shorten / put checkpoints if I did it again since a lot of people seemed to struggle on it.