Set up anybody but the Dutch at the Cape Coast early enough, tbh. The English, the French, the Spanish, the Portuguese, the Genoese, the Danish, the Courlanders, the Russians, the Jews, the Ottomans, the Moroccans. Take your pick. Any state or even people with an interest in trading with India via sea routes would have been a better candidate to plop more people down and see the exponential population growth more notably centuries later. Bonus points if they've got a restive minority, poverty, fear of being pushed out of the Cape by another power, or the complete decimation of the nation save its navy resulting in a population exodus. The Netherlands was just too damn prosperous a state, as has been outlined above. Portugal's probably the perfect candidate. The people themselves were poor, prone to external migration, exported people out of their own settler colonies in the Atlantic, had a large interest in India, and were surrounded by a large power that wasn't too keen on continued Portuguese independence. The TL Portuguese Southern Africa tackles this idea in a very interesting manner, it's a pity the reboot got stalled.