In the Family tab, click on the + icon at the top right, then select "Invite Others", and finally "Invite in Person". That will bring up a login screen.
Once signed in, all the subscriptions will be back instantly, and family members have confirmed that access is indeed restored.
I think we need to tread VERY carefully after doing this. Don't remove this account from the family for any reason, as I believe adding/removing from family has its own 90 day lockout.
I've got a calendar reminder for 90 days....Worst case if I can't get the migration done by then, then I can simply restore that media account as my purchase sharing and we'll be back to square 1.