There you are, before the Fall Gelb the units have been concentrated as they should have been for the offensive, with far less manning the Siegfried Line/in reserve than during the invasion of Poland, as invading France was correctly seen as a lot more difficult task.
Yet in 1939 there were over 40 divisions there, virtually no chance for allied breakthrough.
So it looks like the only window of opportunity existed when... Germany was in positions to invade themselves. Actually makes sense, that's when the army is most vulnerable, concentrated in narrow corridors, easy to cut off by the invading enemy, with gigantic stocks of materiel 'right on the border' etc. Like the Red Army in June 1941.