There are several threads and GitHub issue discussing this.
TL;DR: Generally, it is not possible. Syncthing cannot write to the SD card, if it is formatted as mobile storage.
You can format the sd card as internal storage. Then Syncthing can write to it. But you won’t be able to use the real internal storage as storage anymore.
Some OEMs seem to circumvent the restrictions on SD card, as at least one user reports being able to sync the external sd card.
Mounting an SD as internal storage will cause /storage/emulated/0 to be on the SD card. All Apps will be non the wiser, everything will work as before.
The SD Card will be encrypted (afaik) and only work in that device.
You should never remove an SD card which is formatted as internal storage.
You should buy a fast SD card. I used a 128GB Samsung card in my Moto X, which was SDXC class 10, and should have 20MB/s write and 80MB/s read. But is was so slow that I had forced reboots all the time.
i wonder if Syncthing Android could be made the way that TiBackup is to achieve writing to SDcard. i know TiBackup is a root app but i think it doesen’t matter in this case, just read this tutorial:
[quote=“jackson, post:1, topic:8546, full:true”]
Is syncthing usable in an Android’s SD card?[/quote]
Maybe this depends on the version of Android? In my naivety I set up syncing a folder on an external SD card on one device. This has worked without a hitch for months.
The device uses Android 4.2.2