I would recommend that you plan for 3 weeks (21 days), then delineate the stops you want to make. Head out, enjoy yourselves, and after 10 days turn around and return by a different route.
That will give you 10 days out and 10 or 11 days back. the trip will be more enjoyable due to not being tied to a schedule. If you want to spend a couple of days at a certain site, you can do so.
You have to decide right from the beginning which is more important, the trip or the destination. If achieving the ultimate destination is the important thing, then that has to be your goal. Drive to achieve it.
If the trip, and the sights along the way is more important, plan for that.
Good luck, and enjoy the trip.