If you recall reddits growth many of their communities evolved as offshoots of a single generic community. This made it easier for people to see discussions they normally would not get involved in, and once the posts in a similar category reached critical mass it moved to a sub Reddit.
I think people are recreating their niche communities here but they are floundering since the user base is still pretty small. Maybe it’s best to post to the “big” communities until the time is right to move to smaller, targeted communities?
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.