Sounds like no children and you are doing all of the driving. Many of the major sites out West seem to be 500 miles apart, i'll do that in one day. Other times it's only 100 miles so setting a maximum that you will do doesn't work for me. No offense to Kansas but the best way to see it is at night or from 30,000 feet. Are you going to waste three days seeing it as eastern Colorado is not much different. We do the 1500 miles to Colorado in two days as we've made this trip many times and don't need to see anything in between again.
IMHO A loop trip around the US in six weeks is going to require some days of more than 300 miles. I would make a list of the major sites you want to see and then try to plan a route that does that without driving to many miles at a time, BUT I would not rule out doing 500 if necessary.