I agree that, past a certain size, it becomes harder to enforce behaviour. However that's where I think that the nature of the Fediverse helps a lot.
As long as maintenance of the userbase is done diligently, and no Eternal September changes the nature of the core userbase, people who want a rougher community will eventually migrate to their own instances. It's hard to do this in Reddit, but perfectly doable in the Fediverse, and the migrating users don't even need to break ties with Beehaw.
So in effect, the problem (people wanting to behave in a way not allowed by the community) solves itself. I think that this gives Beehaw way more room to scale than it looks like.