Transfer Old V3 mail file to new M1 Big Sur Mail V8

Unable to transfer previous Mail File (From old Macbook Pro  (13-inch Mid 2009) OS X El Capitan (10.11.6)-Last update available -Mail File V3) to the new (and much improved) processor (To: MacBook Air (M1, 2020) MacOS Big Sur (Version 11.1). Used WiFi connection for transfer.

What did not come over was the set of emails that had been moved from email server accounts to "On My Mac". I have some 8,500 emails (~800 GB) of emails in multiple layers of folders (I have been using Macs for many, many moons)

I used migration assistant to transfer over - All else seemed to work well except the historical mail files on My Mac were missing. Attempted second mitigation assistant transfer from scratch - Same result

Then tried manual transfer: Transferred the old MacBook Pro Library Mail (V3) Mailboxes to a shared drive. Attempted to "Import mail file" from shared drive (Files visible on shared drive. All files are ".mbox")

New Big Sur mail responds with "No valid mbox files were found."

My suspicion is I am dealing with an incompatibility between the V3 and V8 mail files, or perhaps too large??