Hi Simon,
Of the ninety or so counties I have visited, Namibia is my favourite. May I suggest a few diversions to make? Some of these may involve dirt roads, but the lack of rain and high quality maintenance of roads means they are generally in excellent condition. So from North to South.
1. Etosha would have been great, but not recommended as the lions would view someone on a bike as a Interesting new type of prey.
2. The white lady of Brandberg. An eminent French archaeologist in the 1940’s declared that this was painted by Phoenicians since he could not believe that local Bushmen painted this. Set in a splendid setting.
3. Swakopmund. A Bavarian town in the Namib desert on the cold Atlantic coast. This is a must to visit. Ignore Windhoek, the capital and overnight there.
4. Sossusvlei. Pass Oryx on your way to see the spectacular dunes of the Namib
5. Luderitz. Quite interesting, you pass a ghost town, but a long detour to get there and back.
6. Richtersveld world heritage site. Enter it from the SA side. Rugged mountains with unique fauna. Spectacular. Roads however are for four by fours and scramblers. Not sure a BMW can do it.
7. Down the West Coast in SA are lovely fishing villages. If you want to postpone the inevitable ending of the trip in Cape Town, deviate to Paarl, Franschhoek and Stellenbosch. You can look Christopher up in Cape Town. He is working there now and I am sure would love to see you.