Hey @Catija as Sam mentioned, to answer your initial question, what we tend to observe in general with existing users of Discourse is that the staff to member ratio scales very much nonlinearly, with a small number of staff handling very large communities.
In smaller, high-trust communities, the ways we see power distributed more evenly among members has generally been through making more users “trust level 4” or by using category moderators. We’ve also been making an effort over the past year to allow permissions to be granted to larger groups at a more granular level. See for example, this topic: Changes coming to settings for giving access to features (from trust levels to groups)
There are certainly some exceptional cases where communities grant admin or moderator privileges more widely. But for the most part, I think where we see that happening, our thinking now is that we have some gaps to fill in ways more aligned with the above.
In your case, from what I’ve heard you describe so far, my feeling is that most of what you’d like to accomplish could be done so with categories and groups for each PTA, with some additional responsibility delegated to group owners to manage their group’s membership and category moderators to moderate the activity with the primary categories defined for each group.
I need to dig into more of the details of what you’ve shared, but if you get to the point of trying to configure things in a way that would meet the needs of your community, we can continue to riff here on both what general approach makes the most sense, as well as to discuss any specific issues that you bump into along the way.