The answer depends on where you store your (local) music today. You didn’t say if you have internal storage for music files or if you’re using an external USB drive or a NAS.
The answer depends on your choice of file format but most people use FLAC or similar lossless compression. If that’s you, the music files are already compressed. You will generally not be able to compress them further.
I don’t understand that last bit about partially saving the music. If it were me, I’d want a full backup, but I guess you could exclude certain folders or files if you really don’t care about them. Although, if that’s the case, I’d just delete them now rather than waiting for a storage failure to lose them.
Here’s a post I wrote a couple of years ago that discusses one approach to backing up music files that are on internal storage.
Edit: as @Jim_F pointed out, a backup initiated (or scheduled) from within Roon will not backup your local music library. I had assumed that your question was more general in nature rather than relating specifically to Roon’s backup feature. As others have said, Roon’s backup feature only covers the database.