All great 'problems' to have though ! Only you can decide what is a 'must see' for you, but when comparing different mountain routes, it's like comparing National parks. Yes, most have similarities, [great scenery, mountains, forest etc] but each are also very unique in there own way. [Yeah, I know. That's not helping much lol]When it comes to the challenge between the Moab area and the Durango area, we are wondering if some the positive factors related to the Durango area could be achieved "well enough" somewhere else. E.g. Is the Million dollar highway still a true must-see for us WHEN we consider the fact that we have just been on the Trial Ridge Road the day before or so. Aah. This is difficult. :)
Trail ridge road, Loveland pass, the 'Million dollar highway,' Utah scenic 12 from Torrey to Bryce are all spectacular and I couldn't get enough of either, or all of them TBH.
I would try and leave earlier or add a day, again spectacular scenery along the way and there are some nice easy walks to the lakes and mountain scenery not far from Estes park along the Bear lake road.On our last day in Denver we will leave the city in late afternoon and drive to Estes Park. We believe that this will give us a better start for the next day that we will spend on the Trail Ridge Rd,
It looks as though you will be visiting at least 4 main National parks. If so, when you arrive at RMNP you should purchase the annual pass for $80, it would have 'payed for itself' [and some] by the time you enter a 5th park. That's for all occupants of the vehicle included.
I also like Michaels suggestion of going to Monument valley from Durango and 'up.' Near Durango is Mesa Verde NP. Much will come down to the amount of time you want to dedicate to the section from Denver to Grand canyon from your 4 week total. To get to Durango via the 'Million dollar highway', you could use US 50 that goes over Monarch crest, through the Currecanti Recreation area to Black canyon. To get to US50 you would take CO91 at Copper mountain [from I70] down to Leadville, which happens to be a historic mining town and the highest incorporated town in the US, and then down past Twin lakes to Poncha Springs.
The pictures in this report might help you to decide if it's "worth it".