Juche would be alien to the mindset of Afrikaners or other white South Africans. It's a borderline religious doctrine which demands absolute faith in the "Dear Leader" who is incorruptible and can't be criticised--that contradicts Christian religious doctrine. It also requires the whole nation to be united behind the idea of self-reliance, and if these are anything like the 20th century white South Africans we know, that definition excludes the majority of the population.
I could see a black South African version of Juche though, where we'd have a country similar to Zimbabwe. Robert Mugabe always had a certain affinity toward North Korea, including the notorious 5th Brigade which killed thousands of Ndebele people who were perceived to be supporting his political opponents. A "Juche South Africa" (call it "Azania" perhaps) might emerge if Apartheid rule were even more brutal, and in response spawned a successful uprising which replaced it with a black South African government which ended up decaying into a dictatorship which was just as brutal but for different reasons. It would be nominally pan-Africanist in outlook, but probably would be dominated by a particular ethnic group (perhaps the Xhosa or Zulu?). White South Africans likely wouldn't be the only group targetted with violence by the state. This would probably result in a civil war which would most likely lead to the breakup of South Africa and confine the Juche-esque regime to one particular region, although in a worst case scenario the Juche-esque regime would win the civil war and secure control over all of South Africa.
Dealing with white South Africans would be a problem, since that could lead to serious repercussions internationally. They'd perhaps sign treaties with Australia, the UK, the Netherlands, or the US to allow for easy emigration of whites, after they seized all white-owned land and property. Those whites who stayed (i.e. poor Afrikaners) would be in for a lot of challenges--they would be very politically unreliable and likely thrown into prison camps. The ethnic cleansing of white South Africans would likely be a very infamous event in the history of the regime, as it would likely draw international attention.
Unlike Zimbabwe, South Africa is much wealthier in terms of resources (even wealthier than North Korea!), and most importantly, has plenty of ports. Although the conditions of Juche (which is very East Asian/Confucian in terms of its philosophy) are hard to replicate even for a black South African government, since South Africa is very ethnically divided compared to the ethnically homogenous North Korea. As noted, the South African desert is a good place for kwalliso-style camps. Political enemies (which may include entire ethnic groups) or other people disfavoured by the state would be sent here, where many would be required to mine diamonds or other resources until they died.
