Level 5 is a bit much of a spike yeah, swapped it with 6 which should hopefully help. I do indeed have some more ideas for this so maybe I'll make something larger of it eventually.
Just to confirm, (ROT13) fjnccvat n obk sbe n punenpgre (ROT13 end) is a step for level 5 (now 6), right? I might be overcomplicating it.
If true, then I think it is still too much of a leap for the player to take. Maybe it is nicer to have that level near the end where you (ROT13) arrq gb fjnc n punenpgre sbe n obk (ROT13 end) at the front so that the player could have a clearer objective in mind and also has a smaller possibility space to experiment with.