Cool stuff! The puzzles I've seen were neatly designed, especially liked the idea of optional challenge objectives! Wish more puzzle games had those. Have to say that the isometric perspective and the 'saturated' graphics do make deciphering the gamestate difficult, also the 'kitsune gifts' required a bunch of counting which can feel fiddly at times.
Additionally both the undo and the restart button should have much shorter transitions, I am used to near-instantaneous undo and waiting feels like lag to me. Moving also felt a bit slow, so perhaps you could add a 'fast-move' option which speeds up the movement by 3x. Most puzzle games have multiple key bindings for undo as well so that the keys z, u and backspace all work as undo. Maybe even right-click. Saw that you had a key config, but you should consider adding these bindings for redundancy.
The control hints at the start are a good way of communicating the key bindings but not all players will reach the states required to trigger them. I imagine a lot of players will miss out on 'hit f to undo' so it might be good to figure out a way to check whether the hints were shown during the first level and give some alternative mode of delivering this vital information to the player in case they weren't.
Mashing buttons enabled me to skip levels, not sure what happened there but I probs triggered some testing keycombo. Might want to disable that. Restarting the demo put me at the start again even when I did not finish it. This will probably discourage players from returning to your game.
The ending screen of the demo should show the completion rate (# of shards collected/total possible amount of shards). A rudimentary level select for the demo would probs help with retention and encourage players to return to some levels.
Sorry to say, but the 10€ pricetag is a little hefty for me, even though I enjoyed the demo quite a bit. I will add it to my wishlist and wait for some sale (esp since I have half a dozen unfinished puzzle games atm :D).
I recommend you check out the thinky-puzzle-games discord. They are a friendly community of puzzlemakers that are usually happy to share their thoughts and give in-depth feedback.