Welp, I spoke too soon. 
After successfully running on the new Mac core for about a week, this morning I am back to the exact same missing playlist tracks issue I saw after the initial migration. It appears to have cropped up while trying to get around yesterday’s upstream failures on Roon’s side (see Odd message "Metadata Improver: Paused" - #5 by Mikael_Ollars for details).
This morning I rebooted the Mac again b/c Metadata was still stuck for me on the Mac core after restarting Roon so I rebooted the Mac. After coming back up the metadata issue was resolved but the playlists have now reverted back to the same state of missing almost all of my playlist files, and strangely they’re missing the exact same files they were missing during all my failed migration attempts; I can test b/c my 80s playlist always only contains Paul Simon tracks when I’m seeing the failure.
As you can probably tell I’m extremely frustrated by this, but mostly b/c there’s no SLA for support when a system is completely down. Roon is hands-down the best music software I’ve ever used, but honestly I don’t know that I’m willing to continue playing a premium for software that doesn’t come with actual support; I’d prefer to use something like Plex, which I don’t like but has a larger community support base w/o paying a premium.