I very liked the modules system of ACSS 1. There are not too many objects in core version of the world, though it is still a complete solar system already. Then you can add +100 moons of Jupiter from a separate archive file, or add +100 moons of Saturn, then play with all these moons, and once you are done with them - you can remove extra moons from the world and add something else, like +100 object in asteroid belt, and play with those object next. So there are hundreds of objects, but nothing really lags, since you can add and remove objects, there is no need to keep all extra bodies in the world at once simultaneously.
And adding/removing action can be done to current world easily and without any problems, your quicksaves are not affected. You only need to switch orbits of your ships to the bodies, which will not be deleted, to not cause error later on, because one of your ships is orbiting deleted body.