I did search for “weather on the way” in google play store. While they don’t have an android app, I found two similar apps that do a lot of what I wanted: “drive weather” and “highway weather.” Thank you for the search term that yielded results. I like that the"highway weather" app allows adding in rest stops and propagates the change into the forecast.

I will note that neither of these options seem to offer turn by turn navigation. So there is still room for some of the navigation apps to integrate this functionality.