Same situation here... The account that holds all the shared files is the only one that has not migrated yet.... It seems done on purpose.
I suspect (this is a personal guess not an oficial answer) that the algorithm that microsoft are using to chose which accounts to migrate is picking those with the least quanity of files first.
From those waiting for the main account to get migrated and therefore lots of files/GBs, you could assume they are avoiding those, but that does not apply to me. I moved all my files to a new account, the old/original account has now almost nothing in it and it still has not been migrated.
As I said this is my best guess (as a developer), did you empty the recycle bin as well?
My V1.0 OneDrive (still active, but not used anymore), has 41 files totaling 9.6MB and the recyc bin was cleaned out weeks ago. So if you are correct, it would only make sense if they made their migration list/plan from the status that was in play in May 2024 (or whenever they created this mess). Which suggests there is nothing we can do to manipulate the migration.
If I was in charge of the planning in this migration, I wouldn't start with the smallest first, I would start with the biggest first and then migrate all the small ones that the big one is sharing to. This way the delta between all the shares on a group would be the smallest. i.e. hours/days of disruption instead of months.
If you started with the smallest first, then everyone needs to wait for the biggest to start and complete before it returns to normal and that is exactly what appears to be happening here, Most people posting here say "I'm the main user, when will I get migrated", which implies the people they share to are smaller and have been migrated.