No, it’s just the most complex circuit that came to my mind. And I was just figuring out the limits of Discourse. In real life, I have not yet had such a difficult task with many discourses, and I now think that the allocation of a separate discourse only for the SSO task is overengineering. If you have come to this, then you are on the wrong path.
Now I adhere to the idea that you need to go from the general to the particular. That is, continuing the examples that I gave above: you first open a general forum dedicated to all pets, their treatment and food, and when it grows you see the need to separate several large categories into a separate forum (cat owners and dog owners, because these are the largest categories and they do not get along well together and in general they are not very interested in reading about each other’s problems). That’s when you can use your original forum as the SSO provider for the two new forums. They will all be in synergy, but will not suffer from too close proximity.
The second use case I see is when creating separate language forums, when the central forum is the main language and plus several forums in other languages connected to the topic via SSO are installed to it. But this should be done if the support in all used languages is equivalent. And perhaps you should also start with one forum, with separate categories for languages, and when it grows and becomes uncomfortable to communicate, then split into different forums.
The main point here should be automatic synchronization of user accounts between all forums and autologin to all forums / sites when entering one of them.