Thank you for the explanation. I think I have to look more deeply into seedvault and re-read this thread a few times to get it 
On my phone, “Backup” is not visible in the system settings GUI. Can you elaborate what you mean by “start from adb root”?
Can I use seedvault from the adb shell to backup single apks with data to my (linux) pc? I think that would be a viable solution for me, I’ll just have to learn a bit more to get comfortable 