Welcome to the community. As you can see, there are some that are quite helpful and others that are … less so.
I agree with you that there should be a better way to do that. It’s been a while, but I’m pretty sure the Chrome deb file handled all of that for you. I’ve always been confused why every company that sets up their own PPA didn’t do that.
I would do it by manually splitting it up into sets and writing scripts to back up each of those sets. Then you only have to figure out the split once.
I wonder if rsync has an option to do what you are asking for?
It also sounds like the kind of thing the old tape backup software would do. Maybe look into something that can pretend the drives are tapes.
I think they’re expecting thunderbird users to use POP instead of imap, Gmail integration, OWA, or other protocol that expects the mail to stay on the server.
Leaving the mail on the server has been great in Thunderbird since the Mozilla days. I did jump to Gmail web app a long time ago though. I’m assuming Gmail support has improved in the last 15 years?