My towing setup

There will always be some. At higher speeds large vehicles push a lot of air around. As long as the entire rig moves as one unit you have done all you can do. If the trailer is wagging then there are better sway control measures available than friction sway control. Your hitch may be a good candidate for the Reese dual cam set up. The problem you describe is very common and is magnified by the size and speed of the vehicle passing. Your set up presents a very large surface to be pushed by the air being displaced by a passing vehicle. The amount of space between the two vehicles also has an impact. It can be minimized by moving as far over as possible as it approaches. The dual cam system would make the entire set up move as one unit.