Like one or two other commenters, I tripped up at the end with the interpretation of the final message. Is the note "Go back?" intended as a hint for this? If so, I mistook it as an instruction to revisit the previous puzzles and didn't give it any further thought at the end. It's unfortunate because at that point there are no more inroads to work at if you don't see the trick. I honestly don't know whether making the final clue make sense forwards would make this easier or harder (I started suspecting I'd made a mistake as it wasn't legible.)
Echoing the general praise for the game - the closest comparator I can think of is the treetops area in The Witness. Two separate laugh-out-loud moments of epiphany on puzzle 11. The first puzzle was the most difficult owing to a divergence of cultural knowledge, but that wasn't a showstopper.