https://arstechnica.com/science/2017...ent-silk-road/ has an interesting discussion on how some trade routes might emerge. It presents the idea that some of the silk roads trade routes arose not because they were the shortest total distance, but because they were an aggregate of the shortest paths between places where the food was.

I didn't see this mentioned in your links, but a critical insight into sustainable trade routes has to do with energy required to travel the route vs. value of goods. Critters (of which humans are one) tend to use the lowest-energy route available to get to their destination. It's why sometimes you get routes that go around a mountain as opposed to other times you get routes that go up and over the mountain: it may cost more to go around than to go over for certain types of transport.

A bit of research on "Central Place Theory" can be useful when looking at trade routes. It relates value of goods with how far those goods are likely to move. It also seeks to explain relative amounts of small communities to large ones. It's an overly simplistic theory, but it's a good first approximation in many cases.