If you deleted all the JPGs, the textual parts would be there but the metadata sets wouldn’t be complete. PMS would have to download all of it again (Refresh Metadata) in order to correct it.
The safest backup is always with PMS stopped (DB files are static/closed)
From there tar is, imho, the most portable, fastest, and easiest
tar cf /path/to/somewhere/PlexBackup.tar ./Library
allows you to make the backup as well as move the
PLEX_MEDIA_SERVER_APPLICATION_SUPPORT_DIR easily if you wanted to.