Thank you, Doug's Applescripts. https://dougscripts.com/itunes/scripts/ss.php?sp=musicfolderfilesnotadded2 is slowly pruning my Music directory down from 300 GB. It may take a bit but I might actually have a hope of fitting all my stuff back on the Air once it's done...
All my music files are gonna end up being named "Trackname 01.mp3" and part of me hates that but I *never* see the filenames any more.
Also apparently part of how you get this is by dragging an album's worth of mp3s *and* a m3u playlist that lists them all. Which is how most music from Bandcamp arrives. So don't drag the directory any more, future me, just drag the files OR the m3u.
This is a pretty epic file deletion, the script says it found 17.5k files that aren't in iTunes's database, and iTunes says I have 18.2k songs. Still not 100% sure everything will fit on the Air but it's gonna be a lot closer, to the point where throwing away 94 gigabytes of old .ipa backups I will never need might do the trick.
Oh and by the way if you are tired of watching me talk to myself as I try to migrate from this battery-fucked Macbook Pro to my old Air, then hit the ... next to this toot, choose 'mute conversation', and you'll only ever see more of this thread if you actually go to my user page. :)