The road south starts below Denver
Colorado Springs and Pueblo are the useful northern approach. Browns Canyon and Westcliffe can both become bridge nights on the way south. Then the road drops into the San Luis Valley and Alamosa becomes the practical center of gravity.
Denver is not part of this region and does not anchor its map. Browns Canyon is intentionally shared with Central Mountains & High Country because the Arkansas Valley is a natural transition into the San Luis road.
Stay more than one night if you can
The San Luis Valley is big enough and strange enough to deserve time. Great Sand Dunes can be the anchor, but it does not have to be the whole trip. A ranger program one night, hot springs under the stars the next, an observatory or roadside stop after that: the valley can keep changing without changing the sky above it.
The floor of the valley gives the sky room
Long horizons open up across the valley while the Sangre de Cristo range cuts a hard edge into the east. At Great Sand Dunes, the foreground changes completely after sunset. In moonlight the dunes become sculpted and bright; on a dark night they nearly disappear beneath the stars.
That makes this especially good country for wide-field viewing and night photography.
Alamosa is the useful reset point
Fuel, food, lodging and roads radiate from Alamosa in a way that makes it the working hub for the valley. Crestone, Great Sand Dunes, Valley View and the southern extensions become separate night choices from that base.
If the road keeps going south
UNM-Taos is an optional extension into northern New Mexico. Its public astronomy project is promising, but treat it as a developing stop and check the current public schedule before making it the reason for the drive.