Have you tried Syncthing-Fork?
They’re not using the slow Scoped Storage API (from Storage Access Framework/SAF) since they managed to get approval for usage of the restricted but more performant Shared Storage API (via MediaStore with MANAGE_EXTERNAL_STORAGE permission. They’ve also applied requestLegacyExternalStorage attribute for backward compatibility with Android 10).
(I was going to link more sources but new users can’t do that).