I’ve just completed a clean install of Big Sur. In its previous incarnation, I had several mailboxes which were “on my Mac”. They don’t appear in the new Mail, of course. I’ve found them on a Time Machine backup, in ~/Library/Mail/v8/Mac-jbr, and copied them onto the internal SSD. However, I can’t persuade Mail to import them: it says that no valid mailbox files were found. There are folders which look like mailboxes: 4D.mbox, for example, which contains info.plist and a folder with a name that looks like a UUID, in turn containing Data, which has various numbered subfolders.
When I upgraded to Big Sur, the mailboxes “on my mac” still showed up there. There were (are) some oddities: Trash mailboxes “on my mac” and “on my mac” ended up in “Favorites” as well as below. But they all seemed to come through ok.
That is not the way to restore Mail mailboxes (or messages).
Open Time Machine while Mail is the front application. Then step back in time in Mail until you find the mailboxes you want.
If you do it from the Finder, you are just moving data around that Mail knows nothing about.
I didn’t just copy them: I then tried to import them using Mail. Your suggestion won’t work, as the Time Machine backup is from the iMac pre-reset, not the current version.
I wish I could help. FWIW, I had a bunch of mbox folders I had archived away a long time ago. I was just able to import one of them successfully into Mail on Big Sur (11.3.1; I haven’t had a chance to update to 11.4 yet), so it should still work.