Thanks for playing. Varied time is an interesting idea, will try to explore in the future.
Regarding difficulty curve and puzzle 19 and 20 difficulty. 19 was simpler previously, however then I noticed an issue in it, reworked it, and it seems I've overcomplicated it now. Currently the most difficult puzzle in the whole game I think. Great job solving it.
I'm definitely going to add even more levels to smooth out the difficulty.
Color coding or arrow sounds like a great idea, will check that out