I would create multiple levels of simulation – maybe only two levels. In one level, it’s real actors that walk around, bump into things, and so on. In the other level, it’s abstract. The “what do I want to do” bit is still evaluated for a bunch of actors (“I want to buy goods” “I want to raid ships” and so on.) However, there are no assets loaded, no physics simulated, just desires and outcomes. Perhaps you figure out how long a particular desire will take to fulfill – a travel will take X amount of time for Y distance; a purchase transaction will take T amount of time; etc. And just schedule simulation events based on those times.
When the player enters the system, figure out what the simulation is, and re-inflate the actors based on simulation state. If Trader Bob is 20% of the way from point A to point B, then spawn the ship of Trader Bob that far along that vector. If there’s currently a pirate raid going down, spawn those entities in essentially random-but-plausible locations.