Have you seen this? Creating a read-only mailing list mirror
I haven’t tried it out yet but bet it would help you.
The topic is about using Discourse as a read-only mirror of the list (ie everybody has to post using the list, not using Discourse).
But some of the comments mention Discourse’s email-to-forum ability, so it might be possible to have it both ways.
I imagine you subscribe the category’s email-in address to the list (so list emails are sent to to the forum as replies - or new topics), and subscribe a “user” with the list’s email address to to the forum category to receive email notifications (so forum posts are emailed to the list).