Mail cannot save information about your mailboxes because there isn’t enough space in your home folder.

I did the same, reinstalled and migrated from time machine. BUT now have an extra HD-data-data which I believe will cause problems.

Then, you didn’t erase the drive, you erased one volume on the drive.

Remove the - Data volume that isn’t being used.

Alas I was only allowed to erase the HD-data file.

You need to "Show All Devices" in Disk Utility. That would allow you to completely erase the drive as opposed to the volumes on the drive.

The 11.2 GB size matches that of the OS volume which should be Macintosh HD.

You installed the OS onto the original - Data volume which caused it to create another - Data volume for it.

Rename the 11.2 GB one Macintosh HD (or whatever you'd like). It will change the other to match.

I ended up erasing the drive, reinstalling macOS, and migrating from my Time Machine backup.

I found no other way to fix the problem (though there might be).