Thank you so much for your feedback.
You got some really good ideas I would like to implement next.
Specially the undo stack would be something really helpful as dead-ends are frustrating but somewhat required when constructing difficult levels as this leads to a more complex solution.
A level select is also a good idea because I observed people need a lot more time to solve the puzzles as I intended to (around 30 min playtime).
Making the level selection non-linear would solve this issue. Maybe this is the reason many "casual" mobile puzzle games have a grid-like level selection with free choise.
Again, thank you for playing my game :)