Allow shared access to Categories

NARRATIVE:
I have one board that is my main board. It has all different areas I am using Talkyard for like Software Development, research, networking, NeighborNet, personal, Homesteading, Accounting, etc. I would like to be able to have "sub-sites" where that domain was focused on one Category. Like the NeighborNet site board would be at neighbornet.network and only show topics and such for that category. Would keep things cleaner for the user. That usage would probably be fine being the same talkyard instance and the website being hosted next to the primary website.

The other reason might be a joint venture type thing where there is say a primary instance, say the Talkyard.io version, but it links to my Category of Talkyard running on my server. Maybe there is federation between the servers for identity and auth, and the data is actually copied between the instances. So if someone visiting my site answers a question in my Talkyard category, it copies over to the official source as well. This would require some engineering and thought for moderation aspects and other concerns. But could be quite interesting.
One reason to do so is that I might be running AI training on the data, I want a permanent copy locally for history, etc. I know the API might be applicable here but this is a bit different.

Another site might be forum.homesteading.com (not mine BTW) and that is part of my main IvanTheGeek.com system. Rather than have multiple instances all running and separate, the homesteading is available to my IvanTheGeek.com users as a Category, but the forum.homesteading.com users would only have the posts in that section. The reason is that most homestead.com visitors are not interested in software development and so would not be cluttered or bothered by posts from non-related areas.

That idea might also apply when supporting multiple clients. Each client has a sub-section that only deals with their concerns. But the master version has access to all. Maybe permissions would handle this?

SCENARIO:
EXAMPLE: