A very interesting scenario with so mamny ramifications to explore:
- For how long Netherlands and Dutch Cape/South Africa would have been linked?
- How far inland Dutch settlers would get?
- How many Europeans would be living there, more or less than in OTL?
- Provided butterflies remained limited, how this Dutch South Africa, would alter the Scramble for Africa, other imperial powers and even European affairs?
I guess the Netherlands and Dutch South Africa could get as long as it would be regarded ok to consider Bantu population foreigners or on how this Dutch South African state would be organized, maybe relying heavily on indigenous reservations as American countries did to keep natives at bay from the civic life for a while.
Regarding the Trek, it would take another spirit, but it would exist and depending on how big the settlers flow would be, it could go beyond OTL South African borders. That also depends on the next question, which is the total population: Netherlands had a very succesful 20th century, with its population jumping from 5 million to 16 million, compared to France (40m to 60m) or Germany (54m to 80m).
Turning South Africa into a Dutch Australia/NZ/Canada, we could easily envisioned 10 million Europeans/Whites living in Southern Africa by the end of the 20th century, which would make the local history and politics even more complicated than it was in OTL. Also, as Dutch colonial attitudes were in general more regressive than the British, we can see on how that would play.
Scramble of Africa would be very different, as the Dutch would be occupying what was the core of the British expansion. They would probably go for the Congo Basin and we wouldn't see British settlers in Africa as it happened in OTL.
South African gold mines that once produced 2/3 of world demand, would make Netherlands much wealthier and powerful than in OTL, having a stronger presence in the world affairs. Keeping butterflies at minimum, they would be an active voice in European affairs throughout the 20th century, would probably have annexed Flanders, kept more colonies and holding them longer, taking concessions at China, etc.