Why not just have a length requirement and recommend a phrase? “Passphrase” isn’t that common a word, so it merits an explanation. But “your password has to be long, but you don’t need to use special characters, and can use regular english word if you want” is surely easy enough to understand.

> it was too different from every other system they interacted with

Translation: they couldn't use the same standard password they use for their banking, their email (also used for 2fa), Facebook, and this porn site they found by clicking on a pop-up ad.