I feel like Mattermost bridged into a private IRC server (Matterbridge is really good at puppetry these days: https://github.com/42wim/matterbridge) would cover the widest gamut of old hands who like IRC and newer users more familiar with slack/discord/similar platforms, without forcing people onto one or the other. (Discord bridging is also a possibility, but I cannot emphasise enough how absolutely unenthusiastic I am using Discord for anything work-related.)
As for improving the mailing list experience, I think a migration to mailman3 would make interacting with the mailing list a lot more friendly for folks not used to the quirks of mailman2. Hyperkitty (the mailman3 archives renderer / web interface) is a really nice experience for browsing list archives, and has functionality to enable replies / new threads / etc, which are _super_ usable. Again, I think this would cover the widest gamut of users both new and old, whilst still remaining definitively a mailing list and allowing searching of all of the NANOG archives.
Discourse is an utterly dire user experience for a larger community such as NANOG. I'm subscribed to a few Discourse instances - the mailing-list mode just isn't worth using (It does not behave like a traditional mailing list, nor a forum!) and I find the web interface sluggish and fairly unintuitive (scaling is apparently expensive): All of this seems to contribute to a much less satisfying forum experience.