Manual subfolder scanning

You’re right about that.

This should actually work, so I don’t know what goes wrong here.

But to make it short: If you need/want working real-time detection, don’t use a NAS to hold your music library for Roon. Use local (to your Roon Core) storage instead (internal and/or external via USB).
You can keep your NAS to hold a backup of your music library.

Update: Another opinion.