As for maintaining a vector, there are no forces on a bridging the gap between solar systems, so once aligned, it's a straight line. If you can harness cosmic rays, you can adjust if for some reason you needed to. And you only have to get close enough to catch rays from the next star to navigate through that solar system.
Not correct about set the direction and forget it LOL
because a spacecraft can not operate on just 1 thruster.
That would make impossible to change direction. So there are more than 1 thruster. Observe drones. they usually have 4 fans for lift and direction control. Helicopters have 2 fans.
The thrust is never exactly constant because fuel or any other propulsion system never burns at extreme steady state. That is why there is gyroscopic for directional control.
As I explained before, the target 4+ light years away is smaller than a needle point when the spacecraft starts out. No way to set it and forget it. It will need constant direction control for years to come. Even flying to the moon just a few thousand miles away it is not set it and forget it.