I think this may be possible to do with the combination of an additional Pen Set in which the Pen Weights are made heavier, and Graphic Overrides. When defining Graphic Override Rules, when specifying the Override Style, you have the option to override only the Pen color, or both Pen Color and Weight.
But it would be best if you could post a few screenshots of the current state of the file and the desired state as well.
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