I can't use Migration Assistant to restore my TimeMachine backup of a destroyed volume to a new Monterey install (long story, but the TLDR is that it's a limitation of OpenCore). I drag copied everything I could from the Time Machine backup into my new Monterey volume's user folder.
When I launch Apple's Mail.app, all the folders & subfolders I had in Mail's "On My Mac" section are gone. "On My Mac" here means these saved messages are local only to my Macintosh, and not in iCloud or any other IMAP server.
I tried to follow the steps in this 12+ year old answer (copying the old ~/Library/Mail and com.apple.mail.plist from the backup into my new ~/Library). My 25,000 emails appeared to migrate, but all the folders & subfolders didn't. It's all in one big "Imports" folder with no subfolders.
Anyone have any modern solutions or suggestions to solve this problem?