OK sorry, I thought you meant 1000 km of tar as you mentioned tar in your prior sentence, and I hear that argument (1000 km of tar before Namibia) often as justification for big bike. They have their place, but they are definitely not the best bikes to explore off the beaten track.
I replaced XT660Z with 690 exactly because it is too big and heavy (and I'm afraid it would be too optimistic to expect new twin Tenere to be lighter). I got lucky and bought 690 with the rally kit second hand with 2k km on the odometer for R120k. And then spent probably another 15-20, as prevention of known issued (regulator, subframe bolts, Wings exhaust, airfilter).
But again - you need to be honest about what kind of riding you want to do. If you are after comfort, fast speed, two up riding and mainstream adventure touring ( Baviaans, Karoo, Sani and such), big bike is going to be better. You can also do what m0lt3n does and use big bike in tougher terrain like rocks and sand. Been there, done that and wouldn't do it again, but some people like to do that. Still even most of them will be screwed withing few km of deep sand in Botswana (which I would recommend to look at rather than Nam as it is much closer and - quite frankly - tougher riding).
If your priority is to explore off the beaten track as much as possible and you want to have some margin for error, get DS bike like 690/701 or similar, not adventure bike. The best DS riding around Gauteng I'm aware of is De WIldt, and you can use DS bike there on the weekend and then take it for proper longer trip once/twice a year. Another reason for DS bike is Botswana - huge areas of public land to be ridden freely. But you need to tackle tough sand. That (and better scenery and the fact that it feels more or less as South Africa) is IMO why people prefer Nam to Bots - because riding in Nam is pretty easy, while in Bots almost any excursion off tar means tough sand.