But in a lot of cases it wasn't the case that a single subreddit had it all. Many topics had numerous sub's covering the same ground or being more specialised or having different rules.
True. It wasn't always easy to discover "everything" on reddit either.
I find that lot of new people joining the fediverse are unnecessarily worried about servers and instances.
Most of this works organically and you discover places, servers and communities as you go. There's no penalty in subscribing to multiple communities. Or all of them.