Is it possible to change the lineweight of a single line?
No. Not unless it’s treated as a Profile edge. Of course all Profiles will be affected.
There is control over Line weight in LayOut and in LO2021, you can control lineweight by tag in SketchUp viewports. Your profile says you are using 2019 so you won’t have that control but you could manage it with scenes in the SketchUp model and stacked viewports in LayOut.
As a 3D surface modeler edges in SketchUp are intended to define the limits of surfaces.
Ok, thank you!