Maybe the problem is that we have it backwards. Maybe it shouldn't be something you remember and push to the authentication mechanism, but something that the authentication mechanism pushes to you.

Like, what if you pick the corpus of a novel that you've read as your master password. And the password manager uses that novel plus several other novels that you have selected (but didn't read/won't read?) to give you a series of multiple choice selections to determine if you know the right book. Just a few short passages. Preferably with proper nouns stripped out.

You have to select the passages from the correct book for all the multiple choices. That way rather than recall, the memory factor is recognition. Combine it with a non-memorable token which you have to present first in order to even see the recognition factor test and you might have something workable.