Mapping the most gentle route for my single speed
That's 4 days to evaluate all the routes. I'd just ride them. You can spend a lot of time with mapping tools trying to evaluate gradients and still get it wrong. Unless you can find a cycling guide that discusses the issue you will almost certainly not get a good answer. For a motor transport engineer (the people who design roads) anything under 1:10 is fine on a minor road, 1:20 on a major road. They just don't care about short steep sections vs long gentle sections. The flattest route could easily be the one where you come out of a compulsory stop onto a short steep section that climbs less than the 5m interval between contours on a most-detailed map.

One electronic solution would be to use google streetview to survey each route. Look at the angle between the street and the vertical walls or horizontal floors. That will give you a rough idea of the slope of the road at each point.