You look to have a good solid idea of how to go about things and remaining flexible will help the trip remain as smooth as possible, although there are no guarantees you can avoid snow and you will certainly encounter traffic but hopefully not too much. If you like to get on the road reasonably early then staying on the west side of a city so that you are heading away from it in the morning rush would be one suggestion and settling down before the evening rush is another. If the weather is kind I would certainly enjoy a bit of 'down time' and visit the Grand canyon although there are other options such as the Petrified forest, Meteor Crater and small detours into route 66 towns such as Holbrook (Wigwam Motel) Winslow (Standin' on the corner) and if you wanted to, from Flagstaff you could drive through Oatman, home of the Burros.