In the naming I’d avoid hardwiring “instance” as terminology. Maybe make it a systemActor instead to make it more versatile in applicability. That also kind of corresponds to well-known actor systems, like Akka, who use the term ActorSystem for the top-level coordinator of the actor hierarchy on a particular server.
I might also cross-ref my showerthoughts on actor systems vs. AP in: Thoughts on Actor Model versus Fediverse