I think a lot of even not very technical people have gotten used to TOTP QRCodes, and being able to store screenshots of them in password managers. (And having experience in losing 2FA keys that they'll go to some lengths to not repeat.)

I wonder if there's a decent way to encode these private keys in QRCodes? You can jam about 4kB in a high density one from memory? (I know that'd be possible from a developer/technical point of view, but if this were my project I'd want a talented UX designer to have complete authority over how this is presented and explained to users.)

One other idea - maybe implement a Shamir's Secret Sharing mechanism where your private keys get sharded and encrypted to a sufficient number of selected friends, so of you lose your s@ private key it can be re assembled by convincing - say - 8 out of 12 selected friends to give you their part?

Or alternatively - automate a "recovery mechanism" where you set up a new key pair and publish it on a temporary domain/site, and can then ask a friend/follower who can authenticate your identity out-of-band - to export all you posts decryptable with your new key, then put you new key and all your old posts back into your main site.