This needed 100% of my brain power... But I did it I make it to what I think is the final level (the one with 16 carts that doesn't want to stay in place after you got them where they need to go).

Really loved that game ! Note the music doesn't loop, but I got the same bug on my game : in Godot the AudioStreamPlayer Node has an parameter "Playback Type" you need to have it set to "Stream" or else the web don't loop it :)