SD cards work on Android 11, although I think it's because of the "scooped storage" change Google made, not because Syncthing fixed it. It works as if it was internal storage.
On Android 10 and older, Syncthing can only "read" form SD cards (read/write works if you're rooted).
So to be clear, in /old/ versions of Android before they started making apps get permissions per directory, Syncthing can write to an arbitrary SD card folder just fine.
In the newest versions of Android you give it Scoped Storage.
In the intermediate, if you want 2-way sync on your phone, your data has to live in sdcard/android/media/com.nutomic.syncthingandroid/<sync folder1 >, <sync folder 2> which it does not need root to write to.
But for just grabbing your pictures, Android 10 has the stupid limitations on external file system access that Syncthing has to abide by or root over.
On Android 10 and older, Syncthing can only "read" form SD cards (read/write works if you're rooted).
https://forum.syncthing.net/t/android-11-and-syncthing-what-...