Ah, then indeed you need a third-party metadata editor. Roon purposely does not alter local files at all, all metadata is held in the Roon database, never written into the files. The only exception to this rule is if you Export the files for use in another player, when basic metadata will be written into the exported files.
The Nucleus is a music “appliance”, not a general-purpose computer, so you can’t add a third-party metadata editor to it. For that, you will need a PC or a Mac to run the editor software. The editors can access your local files stored on the Nucleus and insert metadata tags into the files.
People here use a variety of editors, Mp3tag, dBpoweramp (both a CD-ripper and tag editor), or SongKong to name but three.
I use dBpoweramp for both CD-ripping (on my PC) and metadata editing, and it is excellent. The developer of SongKong is active on this forum, and understands what Roon needs.
I’m afraid Classical metadata is often of questionable quality - it’s very often a case of GIGO: Garbage In, Garbage Out.
Roon Labs do the best they can to clean up what they are given by their metadata suppliers, but poor quality is the reason for this forum category: Metadata Issues, where we can report errors to them, and they will try and get them corrected with their suppliers.