Thank you for the detailed feedback, and thanks for playing my game! :)
Yes, I also had issues with jumping, it's basic is_on_floor() check in Godot.. I'm thinking that maybe I need something more custom for this kind of a game.
I can't promise if I'll continue developing it, but I really like it myself - so maybe I might make a different game with similar mechanics in future. I am watching some platformer design tutorials etc. in background :D