I used to run my own mail, but gave up due to the risk of me screwing up backups/restoration.
Maddy (sqlite) + https://litestream.io/ might be simple enough to get right.
Mail is one of the easier things to backup IMO. It's just a directory if you use Maildir. Just tar it up with other important stuff on your filesystem you probably also want to save. No need for fancy "streaming SQLite replication."
I recall Litestream citing a ~3s streaming replication lag.
What replication lag could tar-on-a-cronjob reasonably achieve for a moderate 5GB mail store?
Even tar'ing isn't trivial: I've seen casing of people having missed preserving file permissions, making restoration difficult.
At 5G size you could do something with ZFS and tons of snapshots and zfs send.