Synology NAS’ “file handle” monitoring functions are deliberately limited for performance reasons.
Force Rescan is the normal workflow in this scenario, and unless you are adding music every minute of the day, it should not be a burden.
There are some tips about increasing the file handles in this thread, which apply even if you are not running Roon on NAS and just using the NAS for storage:
(under “Getting Roon’s Library to Update Automatically”)
But you need to decide whether or not it’s worth making the changes.