So just to round this out, I run my own mastodon instance. With only one user. Me.
This was mostly because the idea of choosing an instance based on affinity never really appealed to me. I've never really been a "joiner".
But it also has the benefit that I can examine the visibility rules first hand. I can see all of you. And you all can see me. Even though we are technically still separate instances, and neither of us explicitly consented to federate with each other. That's the default.