Looking at it, my persoanl thinking and opinion is that, Implementing Syncthing into NS8 for the purpose of backing up local Data from phone to Server, or PC to server, might not actually be the best solution, refer to questions in this discussion Defining Shared Folder Access Volume - Development - NethServer Community
However, implementing Syncthing to Backup files In Apps into another Location, might be a great idea.
This would be better placed to something like a NAS, otherwise youll be forced to do weired Mappings of NS8 into another NAS box for storage when files get huge.
This is just my personal Observation and opinion.
I could be wrong, and a much clearer picture might help me better understand this aspect.