I use FolderSync. Works perfectly fine for me. But I only backup or update data and I initiate it.
I understand you can also install SSH Helper on an unrooted phone.
Then you can use rsync transfers, perhaps initiated automatically from your OMV NAS whenever your phone is available on WiFi the LAN.
For instance use a crontab script that tests every hour if the phone has been successfully backed up the last 24 hours, if not it checks if the phone is online now, if it is it runs a rsync script to back it up using a time stamped rsync snapshot. And automatically purge very old snapshots. And finally trigger tasker on your phone to tell you that the phone has been backed up successfully again.
Or have tasker trigger a backup whenever the phone connects to WiFi and a connection to your OMV NAS is available.
This seems fairly advanced... Not every user can program something like this, (without wasting a lot of time). So I'll just ask if you already implemented this or know for example the command to 1)check if phone online 2) check if it phone is on WiFi (not just mobile data) 3) Check if it backed up in last 24 hours.