One could argue that the Boer Republics were a more impressive accomplishment than the Mfecane. On the other hand, the Zulu are iconic to Civ.
I'm not sure why America, Canada, and Australia should all be in the game, but not the Boers. I don't see what's so bad with having a lot of Western colonial offshoots, they are some of the most recognizable countries today.
Yes, they would be a good modern age alternative for the Dutch.
The Boers (Afrikaners) are predominantly of Dutch descent, yes.
As mentioned, they also have notable French Huguenot and German roots, and even, (to a lesser extent,) Flemish, Danish, Norwegian, Swiss and Swedish ones.
In a broader context, South Africa makes a good modern amalgamation point for many peoples, since it is composed of many broadly endemic cultures (rainbow nation).
Afrikaner, Zulu, Xhosa, Pedi, Tswana, Sotho, Tsonga, Swazi, Venda, Ndebele as well as the San and the Khoikhoi naturally culminate there. (Not disregarding Botswana, Swaziland and Lesotho)
In fact, the very first modern humans inhabited South Africa, 100 000 years ago. Hence the Cradle of humankind world heritage site.
It is a bit of a stretch to jump on from a non-endemic group, but if you are willing to make that leap then you have even more options:
Malay, Indian, Portuguese (the first 'explorers' to land in 1486), and more.