As for flying to the moon, there's gravity from two bodies as well as acceleration, deceleration, etc.
Bridging the gap from one solar system to another has no forces other than momentum - no acceleration, no deceleration, and only the gravity of the destination solar system. We can calculate the consistent orbits of 100-year comets and they're exposed to gravity from multiple celestial bodies. Plotting and maintaining a course between two solar systems would be less difficult.
There's also stored power, and the potential to have an array that can harness power from cosmic rays.
You said set the direction and forget it! That is so far from reality of jet propelled space crafts.
Gravity? It is zero 4+ light years away.
You must be reading too many space novels. I worked with real physics throughout my career as mechanical engineer designing highest impact in the world machines (cold extrusion of steel).
I will end this discussion by saying no earth spacecraft can make it to another solar system. I am willing bet money where my mouth is. If it was possible, we would have alien visitors constantly visiting earth because there are many older solar systems than ours, and therefore good chance of being more advanced.