The problem with the SyncThing Android app is that it's just a wrapper around SyncThing, which is a Go library, but SAF does not give you simple file descriptors you can use in native code. Instead, you get "content://" URLs, and you need a Java/Kotlin bridge to convert these to file descriptors. That would need to be done in SyncThing itself (EDIT: or some other trickery, because it seems like syncthing-fork made it work somehow).

However, AFAIK, this problem would not apply to the NextCloud app.

The fork is in the play store and works fine for me on Android 15: https://play.google.com/store/apps/details?id=com.github.cat...

I was a bit surprised that the official client suddenly disappeared though.