That’s definitely a case where absolute numbers matter, yeah. I miss the shared-experience subs most, too, though there’s a surprising amount of stuff here already. You might try posting about narcolepsy in some of the broader health [or whatever category you’d put it in] communities, and see who’s there. That’s the kind of thing people might not expect to find at all at this point.
But I think a lot of people are talking about hobby communities, which can be made active by either a large number of people who post rarely, or a smaller number who post frequently, without having to change the overall content very much (I think a lot of people who share any projects could share more projects than they do). The number of people required for “critical mass “ in a forum is a lot lower than people think, and also, a lot more affected by who the people are, and the climate of the community. It’s something I (and I’m sure many others) remember from before massive platforms existed, but apparently it is not obvious to people who didn’t see it. Different experiences, different internet, but I think the essential desire for community is the same, and small communities can flourish in the same way.