Being a colonized by the Iberians probably would help.

Apart that, I think that the dynamic of colonization in South Africa is, from the very start, markedly different from, well, pretty much anywhere else.
The first White settlement was in a area were there was relatively little in the way of natives, and racist objection to mixing with the Natives and/or importing other non-Europeans (Cape Malays) wasn't so dominating. So, you, in the Cape you have the premise for a culturally European-dominated, largely Afrikaans-speaking, and possibly, over time, majority or plurality "coloured" (as in, descendants of Dutch settlers and local Khoi-San peoples) colony in much of what used to be Cape Colony west of more or less Port Elizabeth.
However, then you have the Bantu. They weren't conquered in the way the Aztecs or the Inca were, and I'd argue that, even if interested, the Dutch or the early British could not do that earlier (that just could sort-of have integrated them in the SA polity "better", at least as far as mixed marriages are concerned). As it was, the main Bantu nation in the eastern half of South Africa were conquered by the whites in a painstaking, difficult process where the defeated groups kept some coherence as organized ethnicities if not polities, and the white colonists tended to be quite OK with that. The Spanish in Latin America weren't that OK with the indios clinging to their customs, especially in religion and politics (I know it was more complicated, I'm really oversimplifying here).
When the Bantu polities were brought into the fold, Europe, especially Protestant Europe, was quite big about racism, mixed marriages were largely frowned upon, and, moreover, the Protestant Churches cared relatively little about converting black people. Not that they didn't, but the great role as social unifier across class, race and caste barriers that the Catholic Church had in Latin America pretty much from the start never really operated on a great scale in South Africa, at least not until much later. The Calvinist Christianity of the Afrikaner of Dutch descent especially morphed into something quite unhelpful to this.
So, I'm not sure. Keeping the place Dutch could probably avoid/delay the Treks, and this would in turn leave the Cape society somewhat more racially "pacified" (in _very_ relative terms) so developing much more like a "normal" Creole society; which of course still means a considerable degree of conflict and general nastiness, but probably less than IOTL.
OTOH, in this way you are keeping out much of what is now SA, more or less all of what used to be Transvaal and Natal, most of Free State, and the former Transkei. Maybe these areas would develop as almost purely "Black" countries similar to the rest of Subsaharan Africa, though I expect that Kenyan or Rhodesian levels of settlers (with ensuing nastiness, and probably little intermixing) can be in the cards.
That's quite hypotetical though.