I had trouble with Syncthing on a Synology NAS being unable to create new files/directories and came across this old locked thread which never got answered correctly Permission denied - Synology Diskstation.
For me the solution was to simply give the “user” local group read/write access to my shared folders on the Synology. This group does not allow member changes, and from what I understand it’s just a system group. Hope this helps somebody else save an hour or two.
-B