If you want to sync the files from your Android device to another, you can do that by manually creating the file .stfolder inside that folder after adding it to syncthing. And you should also set the “Master Folder” setting for the folder in Syncthing, so it will not try to write to the card.
If you want write access, you will have to have the current version of Android (6+) with the SD Card set as primary storage. That’s what I do on my Moto X Style.
Android 5 does have the possibility for Apps to write to the card by using a special API, but that API isn’t supported by golang.
Searching this forum will give you threads with more detailed info.