After more than 100 tries and hours on the phone with Senior Support, I miraculously was able to complete the migration last night. I was an iTunes Match user and did not change ANY of my family share settings. Dare I say it, it just worked!
If it hadn’t worked for you previously, it might be worth another try!
Wow, that is wild. Congratulations! I am very excited to try this out again, but I've got 21 days until I can change my iCloud's purchase sharing back to my secondary, allowing me to drop my secondary from the family sharing (which is necessary as I am locked out from switching my sharing account again within the 90 days).
Will revisit this in exactly 3 weeks! Can't wait! Right now I don't even see the migrate button anymore, probably since my secondary is a member of the family sharing.