If we use a 1948 POD to get rid of apartheid, what happens next? A major issue is that for every election going forward, if the NP wins they just vote apartheid in at that point. So what's needed are changes so that the NP can't ever win. The non-apartheid parties have to do well enough that the NP can't govern alone, but would need a coalition partner who would not support them in such legislation. Electoral reform is absolutely essential. A combination of expanded coloured enfranchisement and British immigration might do it, but the UP really needs to be invigorated. Smuts was running out of the energy in '48, and the UP needed new leadership.
At the same time, the UP must still stand up for some form of white control of South Africa. If it moved too quickly to bring in blacks into the government (and in the late 1940s and 1950s, "too quickly" would still be glacially slow), they scare off too many whites. The UP has to do a balancing act of slowly enfranchising non-whites, but at a moderate pace. This will work for a little while, but eventually these slow reforms create problems of their own. You raise people's expectations faster than you can fulfill them. So while I think a UP South Africa might do well throughout the 1950s, but the 1960s we will see more and more unrest as black South Africans demand more improvements very quickly. By that time, it might be too late for an apartheid state to form, but there will be major disturbances and some kind of political solution must be found.
We might see major areas of South Africa given their own independence in a bid to 1) meet demands for greater black political involvement, and 2) allow whites to remain in power in the rest of South Africa. These would be a kind of super bantustans which would include more land and be internationally recognized. We may see most of the eastern half of the country given independence (perhaps with some kind of economic union). There is also the possibility that majority white/coloured areas in the east might be given their independence as well as a separate Boer republic (basically the Free State province + Johannesburg). That would leave South Africa with only (what is now) the Northern and Western Cape provinces and perhaps some exclaves around Durban. These would only be general borders of course, the specific borders would be decided by negotiation.
If South Africa doesn't split, I see very bad political disturbances because you still have black frustration, but without the coercive power of the apartheid state to keep the peace. I'm thinking of an Algerian Civil War situation
If that happens, I expect the rump South Africa to do well. I expect severe problems in any Boer republic. Any independent black states will probably be run as well as the rest of Africa after independence, which isn't too good. But it's always possible one or all of them might surprise and be run as well as Botswana.