Also what is the reason not to use the mariadb tool provided by borg?
I don’t know what tool you mean and can’t find any references online. I do see that Borgmatic allows hooks to run a program like mysqldump before a backup run, but it’s neither part of Borg itself nor has anything to do with streaming data, so I’m still confused about what tool you’ve found.
The advice you’ve gotten is good and it’s what I do. A cron job runs mysqldump, a different cron job runs borg, and I do error checking on both of those as well as occasional test restores.