One complication:
While it is possible to take a bus from La Junta to Denver, it is very time consuming and requires an overnight in La Junta. Greyhound does not stop in La Junta. The state-sponsored bus service does not stop at the Amtrak station, but does stop at the senior citizens' center nearby.
https://ridebustang.com/routes-maps/
One alternative is to take Train 4 from Los Angeles to Raton, spend a night there, then take Greyhound to Denver. Raton has an intermodal station on the edge of its business district. That Greyhound is not an Amtrak Thruway bus.
An alternative that might work well with the rail pass rules would be to connect from Train 4 to Train 5 in Galesburg, Illinois. I have visited both Raton and Galesburg and they are interesting towns.