I think the Afrikaner in the Transvaal/free state won’t treat the black Africans nicely, I could see a lot of them fleeing to the cape or natal (considering there is the orange river the later would be most likely), OTL south Africa didn’t have a multiracial neighbour for people to flee to.
6.5 millions people have emigrated to australia since ww2, let’s be honest, the cape wouldn’t be as wealthy as Australia, and if some go to natal too then it would likely only add less than 5 million white people while including natural growth, I could see it reach 20 with black immigration though.
I wonder, if by the 60s the Afrikaneer Union is going full apartheid, since it may be landlocked in most of its side except the invaded southern Mozambique, could Portugal try to retake it (assuming Salazar or an equivalent is in power), could natal, Rhodesia and the cape either turn a blind eye or even support directly or indirectly the Portuguese since they are the less bad option?