I read most of the google groups posts from the web interface at
groups.google.com, not via emails. Would it be possible to setup discourse to work as a mailing list (as well as having the normal interface, of course), and have it archived by google groups? Google groups supports that, but I don't know about discourse's ability to function as a mailing list.
Also, maybe it's just me, but I was never able to properly navigate discourse forums. The interface is somewhat confusing, though I'm not sure I can pinpoint why exactly. Maybe it's just not being used to it...