Your trip is set at a very relaxed pace compared to many but as always, it's your trip and it's what suits you and how you want to spend your time that is important. An option I would prefer is to visit Arches and Canyonlands NP's even at the expense of the Great sand Dunes NP. I wouldn't spend more than 2 nights in Denver, 1 would be enough for me so I could get out into nature, but again that's me. So if Arches were included, this is how the loop would work out. Denver > RMNP > Glenwood Springs > Grand junction [Colorado National Monument] > Moab for Arches and Canyonlands > Durango > Silverton > Black canyon > [Sand dunes ?]Colorado Springs > Denver.
From Grand junction to Moab you can cut through Castle Valley on UT128 which is a lovely drive alongside the Colorado river in parts. US50 from Black canyon will take through the Currecanti Nat Rec area and over Monarch pass.