imap-backup. It just extracts everything from imap server as mbox. Then that's rdiff-backup'ed to target disks. You can imap-backup it back to another IMAP server if you need to.
However it's better to just delete most stuff so you don't have to back it up. Think my mailbox is about 50Mb.
Doesn't that depend entirely on the storage format? For example if you use thunderbird as a client it has database files that you can copy. Since you say that one of your devices is running arch you should check out some of the email options in the repos. There are at least a few of options that can act as a middleman to bridge between your external accounts and your clients.
> So having a plan to roll back to a stable state might be helfpul. I installed and set up Timeshift for this
Snapshots make this trivial. Particularly in the case of btrfs if you configure your bootloader to always use the default subvolume then rollback becomes just `btrfs subvolme set-default`.