Wife and I did a similar trip and I can offer the advice of ... be flexible, don't push so hard all you do is drive. Your new itinerary sounds pretty good. The 2 day stops is what we did also and it is nice to take a break from driving for a day. There is so much to see along the way that if you see something that interests any of you then take the time to see it.
I went on line and ordered travel planners of all the states that we would be going through and I found them to be very heplful. Stop at the visitor centers as you enter each state too and see what is in the racks, engage the kids to find something they might like to see and talk to the attendants for their insights. Again, be flexible and enjoy the beauty of this great country.
Stop at Wall Drug just outside the Bad Lands, go see the Crazy Horse Monument while you are in the Rushmore area too.
Splitting this trip and a Grand Canyon trip is wise because it gives you an excuse to do it all over again. Plus, there is so much to see and do in the southwest you will need just as much time for it as this trip. Go have fun.
