This works for us:
Step one: Keep your instance civil. No tolerance for horrible people (racists/bigots etc).
Step two: Maintain a vibrant local set of communities free from nastiness.
Step three: Let your users engage with the noise of the fediverse as much or as little as they desire.
We don’t bother with telling our users who or what they can access, and don’t immediately ban visitors based on their home instance. Will that scale to millions of users? Probably not. But that’s a problem for future Nath - maybe.
What would be the point? Reddit doesn’t make any content. They’re just a platform. If they go ahead and paywall subs, those subs are going to have a tiny potential subscriber base. Therefore, they will be less attractive to post to (smaller audience, fewer upvotes etc).
About the only place I can maybe see it working is AskHistorians. And you pay the Historians to answer the questions. Which would of course reduce the amount Reddit takes from the paywall. Doesn’t seem worth it, to me.
Even then, I think the Historians would rather reply in a new free sub with wider readership than take $20 for putting in three hours of work responding to something. They do it because they’re passionate. Not for money.