I think the problem with more immigration to SA in the 19th century, is that without significant further economic development, the migrants are either going to go back home, or move on to another colony or destination.
That being said, like NZ, it would not take much of an increase in immigration to substantially increase the 20th century population.
So perhaps get the Brits to do another "1820" style settlement of another 4-5k somewhere in the Cape. Although looking at Wikipedia, the view seems to be that most of the 1820 settlers moved to towns pretty quickly after the farming did not work out. So you could end up with more of the same, English speaking towns, Afrikaner speaking country.