(Preferably with a post 1872 POD, but that's certainly not a requirement)
This is a 1819 POD:
Have Shaka Zulu be assassinated in 1819, leading to a collapse of the Mthethwa Confederacy shortly after, and an avoidance of the Mfecane. The Mfecane led to massive population reductions in Transvaal and Eastern Free State necessary for the establishment of the Boer Republics IOTL.
[1] The Mfecane OTL dropped the settlement area in the East Free State from 204 hectares in settlement to 13, and the eastern Transvaal dropped from 214 to 80. The western Transvaal was much less settled and only dropped from 17 to 13 hectares in settlement. This obviously had a huge effect on how easy it was for the Boers to take over the area.
ITTL, the native presence in Eastern Free State and the Transvaal will be much, much, stronger and the Boer treks won't get much beyond Bloemfontein (though treks may go north in Namibia and experience varying degrees of success). Geographically, ITTL's South Africa will probably be OTL correlate to West Cape, North Cape, the Western half of East Cape, Western half of Free State, and potentially the southern two-thirds of Namibia, parts of western Botswana, and perhaps an exclave at Durban (the port is too good for some European power not to take it, though it could end up in French rather than British hands). Given that the White population of South Africa is more concentrated (into a region that already had a high percentage) and many African-majority regions are never incorporated, the White population should be at least 2/3rds of ITTL's South Africa (possibly higher). With this dominant demographic position and the Liberal legacy of the Cape, suffrage can be fully expanded to the Coloured and African populations without any fear from the White majority (there might be wealth restrictions for a time, that give way to
de facto Universal Suffrage if the wealth requirement isn't raised).