Best of luck debugging jmb, I don't think I'll be of any more help here - I don't plan on retrying migration assistant anytime soon. Hopefully this helped add some data points!

Thanks Quinn, just submitted. Hopefully this helps drive the issue in the right direction. Case-ID: 22028697

Agreed its probably a similar process. Have you tried the nuclear option of resetting your device?

Created a case - 20000140402724