TL;DR: GoLang doesn’t support Android Stroage API properly so syncthing cannot access an external SD card properly at the moment.
I’m taking a lot of pictures – not only with my camera but also with my mobile – as it happens to be the camera that always available 
But sorting pictures on the phone is a little bit painful for me, I’d prefer to sort them on a larger screen while the result of that sorting should be effective to the mobile, of course.
I’ve had a solution to that for a short time in the past with BitTorrent Sync – bu then, they changed their licensing and dropped some features and my use case could not be fulfilled anymore.
On top of that, a „feature“ was introduced to Android that makes it harder to deal with files on the SD card.
Then, I stumbled upon SyncThing and hoped to get back to a working setup for my photos-from-mobile workflow.
Unfortunately, SyncThing was also suffering from the sd-card-read-write-permissions problem and thus not an option for me.
Last night, I decided to check again and found that it’s still not usable for me 
The situation has improved a little bit as in Android land exists an API not to handle files on the SD card as flexible as required. Bad thing is, that those API is currently not available to syncthing as it has not been implemented to the software stack used there (GoLang), see issue #10588 of Go.
I’ll check back on this regularly as long I can’t find a better solution
Would also love to have sync back to have a very easy way to maintain the set of music I’ve on my phone (and add/remove stuff from there).