Monterey upgrade from El Capitan mail accounts not working

Finally got it sorted. The problem was with the auto complete functionality in Mail, it thought that the only possible connection to my email provider was by IMAP and didn't have settings stored for POP3. This broke the mail import as my accounts were set up POP. And trying to enter the accounts using the Add Account window wanted to set up an IMAP account, not the POP we use.

The solution was to enter an email address with a fictitious provider (I used an email on a domain I knew didn't have email) with the password for the account I wanted to set up, then when I pressed enter it brought up the option to choose POP then edit the email address to the correct one, then manually enter the server details. Press enter and the POP accounts set up just fine. I then imported the mailboxes from my old Mac and now up and running on my new MacBook Air.

Thanks to all the people at Apple Support who dealt with a very frustrated customer, but taught me a lot and made the final setup a lot quicker, and to Tasha at my ISP who said edit it and see what happens.

The email accounts are POP and are kept on the server the accounts download to several computers. Hence the huge number of emails that want to download if I try and set up a new account.

There has to be a way of getting Mail to recognise the mailboxes that have been migrated. It's all there, just Mail doesn't see it.

I've since updated Monterey from 12.0.1 to 12.2 but that didn't help.

Ok, the plot thickens.

I managed to get into Mail by creating a burner Gmail account, tried iCloud first and that crashed Mail and existing Gmail account wouldn't let me in saying the account already existed.

So I get to the import dialog, select the Mail folder from ~/Library/Mail from my old mac which had been copied to desktop, but when I choose that Mail freezes and has to be force quit.

The freezing and crashing doesn't look good here, something is not right.

Would be good if I had access to the Import menu, but I've got to set up an account first and that bypasses everything that's sitting in the Library waiting for Mail to see it's there.