40+ days, 7,000 miles. What should we see?

I know those cities you listed are likely just points along your route, but a number of them are not exactly known as scenic points and much of that route may be vast open emptiness, which may be worth seeing, but can get old after a few hundred miles. When it comes to the Rocky mountain states I feel it is much better to stay closer to the mountains. Places like western Wyoming, Western Colorado, eastern Utah can have their highlights such as Dinosaur national monument, (which honestly is a half a day worth of activity unless you are hiking), but is mostly mile after mile after mile of harsh terrain, with the occasional oasis of civilization, the mountain region has a much higher density of sights and things to do per mile. Much the same goes for eastern Colorado, the Texas panhandle, except there is a bit more agriculture / cattle grazing lands there. To put this in perspective the drive from Amarillo TX to Denver CO up Hwy 287 to I-70 is over 400 miles and not a town over 10,000 people the whole way, and those are few and far between.