I believe the simplest method for tagging many thousands of files quickly [translated: Anywhere from a few hours to a few days, or more depending on lots of variables] is to use WMP by selecting your music folder in Options. Once that's finished you can "select all" and then right click "Update Album Info". This is an imperfect process yet what makes it work in general is that not only will WMP load most of the info if it is in the database, but it also loads in "Composers" data, a rarity indeed. There ends the need for WMP. Once this process is complete go to JR, click on "Audio", then "Edit", "Select All", then "Update Library from Tags" and again, take a long coffee break. Be sure not to select "Update Tags from Library". Hopefully within a day this process will be complete. JR's a bit spotty with regard to bringing in metadata from its own database. At least it is in my experience.
Best, J3