You would certainly hope there to be a high level plan/timescales for such a migration!
Although I wouldn't hold your breath for Microsoft (or most large companies) to provide such useful information 😢
And I agree that if they could group backend migrations on linked Family Plans/Subscriptions that would likely really help reduce the amount of time most people were feeling this pain.
For info: Today I tried creating a new OneDrive folder with files on the account I need the mapped share from, and then sharing with full Edit permissions. As KellyMissey on Pg 11 here had said that worked for them.
But no joy and I just got a web URL to OneDrive, not a proper folder or sync within Windows 😢
Although KellyMissey said they tried that multiple times till it worked. I've only tried one atm