Running the media streaming software on a separate machine is a good idea IF you need transcoding; ie, you need/want to translate the files into another format or a lower quality (for poor remote connections) on-the-fly before serving them to users.
If your clients can play the files just fine as-is, another machine doesn't really add anything except complexity.
Sigh. Spend an hour looking for a solution, fall back to a post asking for help, find the solution 5min later....
For anyone else searching in the future:
From paperless-ngxs settings page (logged in as an admin)
'Open Django-Admin' > 'Paperless Mail' > 'Processed Mails'
Select what you'd like, then delete.