There are three possibilities to use the external SD card without root as far as I know:
- adoptable storage
- folders in the apps “area” (don’t know how it is called), e.g.
/storage/014A-7323/Android/data/com.nutomic.syncthingandroid/files
-
Master
folders (as long as you create the .stfolder file manually)
Maybe, Phone-pictures is a Master folder.