After considering the route some more, I’ve concluded that’s it’s probably unrealistic to make it all the way to Colorado in only 4 weeks — it would require doing 60 miles a day on dirt with virtually no rest days. My new plan, which I think is brilliant, is to rent a car in Jackson WY, drive to Steamboat Springs, and ride home from there. There’s a marked spur off the route to Winter Park, and from Winter Park I can get to Boulder via Rollins Pass.
My only regret is that I will miss Union Pass in the Wind River / Absaroka range, which is supposed to be beautiful. But on the other hand I’ll also miss the Great Divide Basin, and that’s a trade-off I think I can live with!