We did consider Discourse and phpBB as migration options, but it seems there is no solution that will make everyone happy (there already is a forum in parallel to the mailing list actually). Many people like the mailing list concept and like to manage all this in their inbox.

Obviously switching to a self-hosted mailman is an option too. But having no access whatsoever to the group, migration is kind of a pain since you have no way of even contacting everyone that was subscribed.