As a casual sudoku enjoyer, this game is great! I love the art and the soundtrack and basically everything about it. It feels super cozy and cute ! The only little nitpick I have is that one bug when you drop in the last one of a kind.
Something you could add in the future would be the ability to take notes (similarly as you can with sudoku) especially if you want to add in more complicated puzzles.
What I really appreciate are the little extra rules you added in, especially the fact that there were visual cues for them afterwards.
So overall you did really really well, great job!