Why would you want a mail specific stack of hosting, storage, indexing and frontends? If it’s all plain text anyway so the regular storage solutions for files come a long way.
Because email has metadata. From, to, sent date, subject, etc. Plus attachments that may be binary.
rclone & restic work okay together to create backups in a Google drive mount. There are “issues” with backing up to Google drive since it doesn’t guarantee file names are unique which is… a choice… but it should be reliable enough.