Nomadic Nomadic, I know you're using a Synology NAS and dealing with Seedvault for your backup, so I don't know if this really will fit for you, but I thought I'd put it out there.
I recently started using an app called Syncthing. It should not be considered a backup software, but it's great for synchronizing data to another, or multiple devices. For me, that's perfect. All I care about is my data. This app allows you to pick and choose your folders and files and send them to another device/s for storage or access. If my phone dies and I lose all the apps, I don't care. I'll rebuild it from scratch. It's the data that I care about.
The configuration is fairly technical and is not a simple install and go type app. But once I got the configuration right, I haven't had any problems with it. And as for what you're looking for, it works on a local Wi-Fi.
There are two components basically. You have the agent that sits on your phone and then you have an agent that sits on the device you're sending the backup to. In my case, it's a Windows laptop. What would be nice for you is if the app would install on the Synology NAS and I don't know if this would work in that case.
I downloaded it to my phone from GitHub and I got the Windows installer directly from their website.
Maybe check out some YouTube videos and see if this fills your needs.
Hope this helps and good luck.