The British will hand over the Cape colony after Napoleon
Natal wiil be thier own half way station to India
The ''Grote Trek'' probably will occure, since the Netherlands after Napoleon is a centralistic governed Monarchy and later constitutional monarchy, very different than the VOC rules, so the Boers are subject to laws, rules and taxes, which soner or later they oppose. These ''opression'' from a gouverment will be the same as British rule OTL and there for trigger a ''Trek'' to the East and North.
Interesting it will be how this trek will conflict with the British colony, call it Natal.
suppose the Boers avoid this and do more or less the same as OTL, only dutch rule will follow after them. When Diamond and Gold are found, most likely the dutch gouverment will react more or less the same as the British. Most likely more subtile, considereing there no Boer republic founded.
The Brittish would let this happen. The only reason they were intersted in South Africa and Later Rodesia, was because of Cecil Rhodes.
I think a dutch south Africa would have borders almost like present day South Africa, Zimbabwe, Zambia and Botswana, without a larger Natal, as a former Brittish posession.
The British will focus on North Africa, and who knows dig a Suez canal a decade earlier.
The British will still became super power of the 19th century, only the dutch thanks to the gold and diamonds of South Africa a bit more richer and who knows more influential.
Perhaps a Dutch gouverment spent a part the gold revenues on Battleships and dreadnougths, and with this substantial fleet get dragged in to one sort of European aliance.