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.