A loop would be better on your timeline.
My immediate thought was that of Michaels, create a loop trip that would potentially less miles with more time to enjoy your many options. Las Vegas could be a start and end point as well as Phoenix, you could check the cheapest airfares. If you do not wish to drive for more than 3 hours a day, (which is something you would have to just to get from Phoenix to Denver via the Grand canyon on a direct route) then the loop would be quite small but with (more than )enough to see on your timescale. One such option would see you go from LV to Zion NP >Bryce canyon> Page AZ (Lake Powell, Horseshoe Bend, Antelope canyon)>Grand canyon>Las Vegas. You could drive an old part of route 66 through Seligman and stop at the Hoover Dam without adding much drive time. You could also detour to Monument Valley and/or Sedona but that would obviously add more driving time.
If you are set on Phoenix to Denver then Grand canyon, Monument valley, Moab (Arches and Canyonlands) and Colorado National Monument would make a nice trip, or from Monument valley >Mesa Verde > US550 (aka Million Dollar Highway) through the mountain towns of Silverton and Ouray to US50 and Black canyon, Currecanti Nat Rec area and over Monarch pass to Poncha Springs. Perhaps visit Leadville on the way to Denver from there.
Share your thoughts and we can help put something together.