There are more 
. People seem to often sync folders created in the root path on the internal storage by other apps (e.g. WhatsApp, etc.). Those are also fixed and cannot be moved.
In my case, the main problematic folders containing a large number of files are a) Joplin (a note taking application), which for syncing purposes stores thousands of tiny MD files in a single folder, and b) Radicale with its thousands of tiny ICS files (used with https://forum.syncthing.net/t/how-to-sync-contacts-and-calendars-between-desktop-and-android-without-root/21326).
Not sure if Joplin would be able to utilise the Syncthing app if it acted as a document provider. It wouldn’t help with Radicale for sure, as this one is used via Termux which needs direct access to the folder.