@falkerie71 I generally backup my phone to my main NAS and have syncthing configured to Sync my main NAS to backup NAS. My primary NAS is now discontinued WD ex4 and backup NAS is truenas running as a VM on proxmox.
The biggest issue I have with auto backup is that it backs up all the photos including the auto downloaded media from Whatsapp that I have no need to retain.
I also use Syncthing with SyncTrayzor to sync files between my devices! Doesn't Syncthing have the "ignore patterns" function? You can use that to ignore certain folders like your Whatsapp folder I think!
@falkerie71 I have honestly not played with it enough to see if certain folders on my phone can be ignored. Would be great if there is an option. I was more like it works and I am not going to touch it ever again :)