Hello again! Gave the demo a playthrough on easy, so hopefully you didn't hide anything new behind beating Hard mode.
The little illustrations between levels are good, but did they increase the amount of time spent between levels? I felt like they decreased the feeling of flow as I was zooming through the early levels, so you might want to make them optional. The illustrations are on screen for such a short time that they're kind of hard to get a decent look at; while you could increase the time they're on screen, that'd be at odds with the sense of flow, so I'd recommend not doing that. Instead, I'd recommend making the background simpler so it doesn't distract the eye, and moving the illustration to the center of the screen so the player's eye doesn't have to travel as far. Something more complicated but cool could be a system to automatically show the illustration right on top of where the exit door for the level was.
Defaulting to full screen felt like it made things surprisingly harder for me just because things were visually further apart and I had a harder time seeing my own position and my jump target at once.
In terms of actual gameplay, the only change I noticed was the timed spikes on level 10, which is a good change I think.