i started by switching accounts mailadresses for all regular services ( also switched to duckduckgo mail forwarding and set the mailbox.org address there for normal Website accounts). Then i just included mailbox.org account in my Thunderbird Konfiguration and with this just drag and dropped the Mails Form Gmail to Mailbox folder. Only downside to this is that those Mails would not become encrypted in mailbox Servers in contrast to normally received Mails. i would advise against mass forwarding old mails, because it would make mail search much harder as they would get tagged with the new receiving date instead of the original one (this is hard for yearly tax return notices if you are looking for orders and receipts received by mail in a particular year), same with the Sender (now your old mailadress instead oft original).

i can recommend mailbox.org and ente.com