I started doing some thinking for someone who wanted to have thousands of multisite sites with a single “main” Discourse site as the authentication source for all of them.
It seemed possible, with some work (and much less work if you have no more sites than you’re willing to configure by hand).
Having the same data on multiple sites is complicated (there has been some discussion about doing this with the ActivityPub Plugin), but you could host the everyone-cares stuff on the main site.