I mean, really, why would it be encrypted?

[Mark Knowles]

Is there no middle ground then between what we should have cracked already and nonsense?

Quite honestly? No, there isn't. Because anything in between would have been a leap of genius to a level above our own 21th century knowledge.
It's impossible for a 15th century cipher - as we know it - to baffle us, yet still retain retrievable knowledge.

Prove me wrong, people!

The Trithemius cipher sounds non-trivial though I haven't studied it:

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