What’s wrong with using the default backup functionality of mailinabox? It essentially includes all of the userdata and you can import it to another box relatively easily.
@murgero, do you know if there are there any immediate downsides to enabling Nextcloud administrator to allow for software module installs; apart from any issues arising not being officially supported by MIAB?
I am using BORG to move the backups that MiaB makes to a directory inside my lan. I have a writeup if anybody is interested (the borg share is the only thing that had me worried - I setup SSH such that the MiaB can only write/append). Works very well ATM. The only problem I have is that I do not think that the calendars are being backed up (All calendar entries for one user (me) gone)