The Voyager quest as we designed it is not a replica of the real mission indeed. It's been designed as a building efficiency challenge (hence the mass limit).
Actually we even forbid fly-bys because it would make the mission easier: each fly-by is a gravity assist opportunity, and you would only need to aim for Jupiter to accomplish the challenge. It could even be done by aiming for Venus, if you know what you're doing you can chain gravity assists until you have enough energy to leave the solar system.
In real life things were much harder, without gravity assist none of the probes could have left the solar system.