Hey, thanks for the comment :)
Level 21 (and 33 for that matter), are definitely on another level of difficulty than the rest of the levels.
The design process is actually quite simple, and i thought about adding an actual level editor: You simply re-create the shape you have in Level Blocks (the gray ones), and copy/paste, position/rotate them so they dont overlap. Do that a few times and you got a solid chunk of level. Add some "Easy to place" pieces on the outside of that. Test some times, adjust accordingly.
When you have all levels, rate them by difficulty and put in a fitting order.