I've set up a local mail archive with just dovecot + imap plugin. you don't need the full mail server with postfix and whatnot, as it's not intended to send anything anywhere, or even receive anything for that matter. it just sits there ready to be searched with thunderbird, no need for other complex solutions you've mentioned.
why not
btrfs send | btrfs receive
? is there some advantage to rsync?did you hotswap the drives after each
btrfs replace
or shutdown and then swap?what's your host OS and do the drives spin down if inactive?
thanks for the writeup!