Navigate on a line

Hey there,

i am looking for a reliable method for flying the airplane on a specified line with a companion computer. I already tried attitude control and to give a lot of targets on one line in guided mode, but both of the methods are not very precise. The mistake it makes is around 100-200 meters. But when flying to a waypoint the plane can reach it with quite high accuracy. But for updating them, the whole Mission has to be uploaded every time which is slow.
Is it only me or is the MAVLink Protocol not really optimized for controlling airplanes in such a way. Isn’t it possible to set for example a target course over mavlink?
How would you solve this situation? For me it seems like there are changes on the pilot itself necessary for this, am i correct?

Paul, did you find a solution to this? I am trying to do something similar with Plane and haven’t found an elegant solution yet.