Many German-Americans in these places and elsewhere did go to fight the Nazis. That does not change that these voters (not all German-Americans) voted out of opposition to Roosevelt’s anti-Nazi foreign policy and continued these patterns while their country was at war with Germany. This is not a ‘bad historic narrative’ but a fact.
So you claimed that Nazi-sympathizing German-Americans drove
a lot of that rural Midwestern support for the Republicans
which is a pretty sweeping claim. Not denying that there were Nazi-sympathizing German-Americans, perhaps especially in these areas, but as Yankee alluded to, it seems reasonable to assume that German-American immigrants and their children would be especially less likely to support American involvement in another calamitous world war in which their country of origin was on the opposing side.
Not saying they were right to oppose it, but that didn't necessarily make them Nazi sympathizers.