How can I solve "Cannot Start Family Sharing" error, when trying to set up family sharing?

On my iPhone I go to Settings -> Apple ID, and click "Set Up Family Sharing", and this error appears:

But... I am Jon, and I'm not familiar with any family sharing setup.

And my Mac says basically the same thing, when going to System Preferences -> Family Sharing (via Catalina 10.15.1):

I've called Apple Support about this but they say I need to log in with the iCloud account my primary account is associated with, but I've no idea what account that could be. As far as I'm concerned I have one and only one iCloud account, and I haven't made any account changes in years. Apple Support insists they can't look up more information.

So how do I use Family Sharing in this situation? Or even just debug this? Apple Support insists there's nothing they can do so I'm not sure what to do next, anyone got any ideas??

Oh.

Okay, I found a hint in an email from 2012 (!) when MobileMe transitioned into iCloud (remember that!?), and I guess I had another account back then which for reasons unknown I apparently family shared to my new account 🤷‍♂️

My strange past behaviour aside this looks to resolve my issue (waiting for account deletion to go through before I can know for sure).

If anyone else comes across this: https://appleid.apple.com guided me towards my long-lost account, and https://privacy.apple.com is where I put it up for deletion.