The second part of that is hard/expensive, but it is a path forwards to some level of discourse between the people using these subreddits and everyone else. Driving these communities to different sites means that people get in much stronger filter bubbles, that's probably not a good thing.
To take the example of /r/conservative on reddit right now, the subreddit currently bans anyone who says anything at all critical of Trump, and mostly only let's people who have a history of saying supportive things talk. That's not good for reddit, or the country as a whole. But if reddit just removed the current batch of moderators and those rules, and had someone enforce some reasonable standards of discourse but not enforce politics, I think it could turn into a force for good.
To put it in your own terms, my preferred solution is to try and make it possible to "have intellectual discourse with people who are fabricating reality or deeply believe in conspiracy theories", instead of just making it so those people go fabricate their reality elsewhere.