There were sth like 10-15 thousand Voortrekkers in the 1830-1850s and even if late 19th century Boer precentages in the Transvaal and the Orange Free State were much higher than today the actual numbers still did not exceed several tens of thousand. Wiki gives a total of around 450 thousand whites for the two republics combined in 1904 but by this time the Uitlanders made up at the very least half of the population. I couldn't find a reliable linguistic break-down, but most sources suggest a clear demographic dominance of the British, even 2:1 ratios or more.
So imagine a more numerous Boer community. A POD could be somewhere at the beginnings of Dutch settlement in South Africa; add several thosand more immigrants for whatever reason and in three hundred years time you might have a Boer community twice as large.
Would a demographically more secure Transvaal have fared better in its dealings with the British? An undisputed Boer dominance might have made the enfranchisement of the English-speaking immigrants less of a perceived threat, avoiding a major cause of the Boer War. Without such a pretext I think it's unlikely the British would have gone ahead with a brutal agression; with a little more diplomacy on both sides maybe the Boer War could have been avoided altogether.
Would then the Boer Republics peacfully join a negotiated South African Union eventually? I don't see that happenig too easily, but than the existence of the highly unstable Republics as independent in the long run seems equally unlikely. Still, if they would survive until 1914 the post-war situation might be more beneficial for their eventual consolidation. Or not. I can't decide.
Also, on a somewhat unrelated note, do you see any way in which additional Boer Republics could have been established in what is now Zimbabwe? When the region was conquered by the British South Africa Company around the end of the 19th century it was completely subdued in sth like a decade and with relative ease, despite the Matabele Wars. With more Boers several decades earlier maybe what became Rhodesia could have been closed down for the British by Boer statal formations for a while. Some vague notes in certain dubious sources hint towards some actual Boer activity beyond the Limpopo; according to Conan Doyle there is supposed to have been a planned trek northwards sometime in the 1850-1860s, which came to nothing though. So what if there had been a trek north?