Thanks for playing and the detailed feedback! Really glad you enjoyed it. And yeah, I made level 6 too difficult.
Itch’s fullscreen option should be there. Look in the bottom right corner for the 
Or perhaps you mean how the aspect is constrained? It’s too much of an advantage if you have an ultrawide (as I do) and can see 3x the distance out. However, if you have a portrait-tilted monitor, you will benefit from the extra height if you go fullscreen. That’s a modest help if you manage to build very high.