Well, I think that "diabolical hard" is a bit of an overstatement here. ^^ I enjoyed playing this. Clean, minimal look. Fitting sound effects. Really well-done tutorials and every feature I could wish for in a puzzle game: a button to show the occluded sides of the dice, undo and restart. If anything I would've wished that one spawns in front of the level one just finished/quit. But all in all really nice job!
Greetings from another Godot enthusiast!
Any reason why you didn't provide a HTML5 version of the game btw?