Harnaś, thank you so so much for this extensive and helpful feedback!
What resonated especially with me is that this could be a relaxing game that could be more focused on sensation and exploration of a cute world (which then would be especially well paired with cute relaxing music).
One idea I might explore is to see if I can either use procedural map generation to create an endless stream of beautiful cute dioramas or to create a large world up front and allow players to glide around more freely.
I have deliberately used deception in my level design in levels like 1-10. I am sorry that I have made you feel dumb and exhausted. :D Overall, I did find it hard to make levels that are not too easy or too hard. I think it is safe to say that I have made the game too difficult and need to make progression easier.
This being my first puzzle, I was quite surprised how little tweaks to the game's rules would have an immense impact what kinds of levels one can express in that resulting "puzzle language." For example, my girlfriend is annoyed about the mechanic that you first sink before the lift of a cloud is applied, which means that you cannot U-Turn under a cloud if you have an elevation of less than 3. In a way it's not realistic or intuitive. When I changed the game to add the movement sink and the cloud lift into one, sevreal levels completely fell apart as you could basically choose your height freely and then travel across the map.
Have you seen or used the help button? If so, have you found it useful?
Without spoiler, have you clicked the "Watch Tutorial" button in the "You've run out of lives" screen?
Congrats on making it to the second world. And thanks again for playing and your input!