I liked this demo! It feels like you've got a really solid grasp on what you want the game to be, and the puzzles were all excellent.
One minor complaint, regarding the restart system: by default, the control key restarts the level, though 99% of puzzle games (that I play, anyway) map this to the R key. I suppose this could be a problem if the player accidentally presses R, since it's so close to the other keys, but I think it wouldn't be such a big deal if you put up a "Are you sure you want to restart?" screen where the player could cancel the restart. Baba Is You did a similar thing with its restarts, and I think it worked really well.
Anyways, I think you're on the right track, and I'm legitimately excited for where you take this game next!