CPT-WDH
NdG, perhaps you want to be in Upington but otherwise it would be a lot shorter (nearly 500 kms) to travel from CPT to WDH via Vioolsdrif. (1482 kms according to Mapsource). Count on overnighting at Vioolsdrif after about 8 hrs driving and then through to WDH after a further 10 hours.
If you want to be in UPT then an alternative route would be CPT Springbok-Pofadder-UPT. (1908 kms according to Mapsource). It is tar all the way with relatively little traffic after Springbok. The Pofadder-UPT stretch is scenic but slow in parts where it follows the Orange River. A nice detour is the Augrabies water falls between Pofadder and UPT. One can also turn off at Pofadder and go down to the river to Onseepkans. (This will eliminate UPT from the route). You can cross the border there and enter Namibia at the Verloorsdrif border post. The road between Pofadder and Onseepkans is very poor gravel (about 50kms). However the gravel road on the Namibia side (about 104 kms) is very good and scenic. You will then pick up the tar road again at Karasburg.
Have a good trip.