You may find the trip report in my signature to be a helpful read. It is long, but you should find it interesting since it is similar to what you have planned. Our trip was 40 days with 2 kids.
If you don't read it, I would suggest doing the Utah parks since you drove all the way to the GC. Zion, Bryce, Capitol Reef, Arches...were all incredible places. It seems that geographically a visit to GC would include those as well. Maybe consider Tetons and Yellowstone on a different trip to maximize efficiency of miles driven. I know this suggestion isn't the question you were asking, but it is worth consideration.
Definitely cut GC to 3 nights. Not sure what you're going to Durango for, but the train ride was worth a 2 night stay there. Mesa Verde was a disappointment to us...one day was enough.