I don't usually play Sokoban games, but I gave this one a try and I would say the difficulty is just right enough to go through all the levels without getting stuck for too long. Yeah it took me around 800 moves to figure out that final level, not the sharpest knife in the drawer, but we got there at the end. The fusion mechanic to pull boxes made the game a lot easier since you don't have to worry about pieces getting stuck at corners.
So yeah this was an enjoyable game with a good amount of challenge and quite some content for a 3 days jam. Nice work!