In don’t use Gmail but I use offlineimap which would do what you want. You can then use the files with thunderbird as a regular mailbox but off your local copy

In don’t use Gmail but I use offlineimap which would do what you want. You can then use the files with thunderbird as a regular mailbox but off your local copy instead of gmail’s. I have it run with a systemd timer at regular intervals and it’s backed up with borgmatic at regular intervals using another systemd timer.