Try using Foobar2000 for any mass changes. It's a great audio tool and we've even started a guide here on Techspot. Check it out here: https://www.techspot.com/tweaks/foobar/
It's just getting started but I'm sure it'll be growing soon.
GX Transcoder is a free utility to convert between a lot of multimedia formats.
I still think that MP3 is a bad "library" target. Use a lossless format for the "archive" and connvert to MP3 or whatever as needed.