To close the loop here I figured it out. The missing piece was fbind:
https://forum.xda-developers.com/apps/magisk/module-magic-folder-binder-t3621814
Download that from magisk, run rclone mount drive: /data/media/0/Movies
fbind -b /data/media/0/Movies /data/media/0/MoviesM
and Plex can see all the movies in /data/media/0/Movies and play them flawlessly! Good luck!