> I've heard anecdotally that after 4 kids, having more doesn't really require much additional effort because the older ones can be recruited to help with the younger ones

I've heard anecdotally, that after four (sometimes two or three, depending on who is describing it) kids having more doesn't require additional effort because there's a limit to available effort, and you're just into how it is divided at that point, effectively transitioning from K to r strategy.