macOS Mojave: Mail accounts not appearing after clean install

Question

I've just finished a clean install of macOS Mojave on my mid-2012 MacBook Pro, which is on the list of supported devices. I tried recovering Mail by copying the following folders/files from my TimeMachine backup and putting them in their respective locations on my new Mojave install:

• Mail content (~16GB for me): ~/Library/Mail/
• Mail preferences: ~/Library/Containers/com.apple.mail/Data/Library/Preferences/com.apple.mail.plist

I then fired up Mail, upon which it needs to "optimise my accounts" etc. (the folder in ~/Library/Mail/ called V3 changes to V6), but then it opens into an empty window. None of my previous accounts are anywhere to be seen. The files in the Mail/V6 folder are still there.

Restart of machine after copying but before opening Mail for the first time does not help. I tried removing the ACL that TimeMachine normally adds to files, using $ chmod -R -N ~/Library/Mail/, but this made no difference.

Does anyone have any idea what could be happening? Importing the mailboxes results only in folders under "On My Mac" while the accounts I had were active IMAP accounts.