From Cortez, Colorado you can take the Western Express to Pueblo.
In Arizona you can take Old Route 66 from Needles to Williams -
Then Grand Canyon, Cameron, Kayenta, to Four Corners.
(US 89 has a detour onto US 160, expect moderate-plus traffic east of Tuba City.)
That means getting to Needles, Calif.
You are aware that the Mojave is still really hot in September?
And that Santa Ana winds can be like being in a convection oven??
You can start in Ventura and have an easy, gradual climb to Santa Clarita -
Or you can work your way to Santa Clarita from Santa Monica thru metro LA.
Then you can follow Soledad Canyon Rd to Palmdale and cross Antelope Valley
(Palmdale Ave and El Mirage are better than Hwy 18 - but pretty bleak)
From just north of Victorville you can catch Old Route 66 thru Barstow to Needles.
You will need to research limited services on Route 66 -
And, as I said above, it will be hot.