where are the big inaccuracies coming from
In some cases, wiring. I can "swing" my vertical card compass by 10 degrees just by revving up the engine, or operating the taxi/landing lights.
The engine prob99 needs to be running, whereas most compass swinging I see done doesn't do that.
One could indeed argue that an accurate heading is not needed if using a GPS, and that is true, but I quite like getting things right
so surely you can only reference compass to GPS in nil wind?
No; see my original post.