"On My Mac" Mac Mail Folders and Contents Did Not Survive Migration in Upgrade to Catalina

Upgraded from Hi Sierra to Catalina. Used Migration Assistant to transfer from Time Machine backup. Everything migrated perfectly except Mac Mail, where it appears I've lost years worth of emails. Had hundreds of folders and subfolders. I think most of these were stored "On My Mac". Eveything else in Mail migrated successfully.

In Catalina: User > Library > Mail, I have folder "V7" with several subfolders like "AD35D528-E15A-407B-A1B0-0BACEA83C0B8" some with contents, others with "Archive.mbox" and "INBOX.mbox". There is also a folder called "Mailboxes" that has about 30 files like "Clients.mbox". etc. Clicking any of these gives me subfolders with names like "4FAB6DCB-E159-4D7B-A0A0-22949C7EC894", then subfolders with single digits, most of which are empty. In few cases where there are contents, they are named "filename.emix". When opened each of these opens in Mail as a single email.

The .emix files seem to be mostly older messages and appear to be intact. It may be that all or many of my previously stored folders and emails are here but it will be a nightmare going through hundreds of folders and subfolders to retrieve them. So, I'm hoping there's a solution where I can try to retrieve/import all of them into Mail. Thanks.