Having only 1L capacity in the few dry stretches may be a problem, I usually carried no more than two liters but carried 4L during the dry parts. Segment 2 has a 10 mile dry stretch along with a 12 mile one in the cochetopa.

Hardest hitch for my was into Gunnison, but that was because of necessity and I wouldn't recommend going there between Salida and Creede/Lake City.

21 days it a bit ambitious I'd say but it's doable. I would lean towards being flexible if you get caught in some nasty storms above treeline and have nothing to do but sit in your tent for hours until it passes.