There are so many different factors at play here, that I believe an ACCURATE app like this would be very difficult to develop. You already mentioned some of the complicating factors in EV vs ICE, weather (not just temp). But then there are even differences in between model vehicles of each type.
It also would depend how you define efficiency. A highway route may use less fuel for some cars but if it adds miles you are wearing the tires, etc faster, plus just adding mileage to the car. Then again a non-highway route would add to break wear, etc.
In my opinion, the most efficient route is almost always the shortest in mileage. However, if you plug this into an app or google maps it will take you on the very shortest route. Maybe adding significant time to your trip while cutting as little as 1 mile on the distance.
I find this a difficult problem that I think about a lot, owning a Tesla model 3 SR. But I have no illusions a good app will come anytime soon to help.
I often try to be smarter than the car and use a more efficient route than it suggests. But occasionally I am burned by this because I am unaware of accidents, road closures, etc. that the nav system is.
Anyways, the best solution at the moment on a semi-long to long trip is to look at the map yourself to find what appears to be the best hybrid between the shortest route while utilizing as much highway as possible. Then if google maps doesn't show this route as an option, simply add a waypoint that forces it to, then see how much your estimated time changes. This is more work than I care to do daily, but for longer trips its my process for now. (Tesla nav sucks in this regard as they don't allow you to manually change a route easily).
