I never placed much trust in the South Africa story. It's one thing living quietly in the UK and raising your family and quite another - living in a foreign state. What official part would have Harry have - General Governor? I can imagine just how well *this* would have been received. And the security costs would have hit the roof. I remember there was some security noise during their South Africa trip. I can't recall the details but anyway, there would have been much money involved in arranging their security, much more than in the UK.
But this is a secondary matter. I just can't imagine a British prince and his wife basically using South Africa as a learning ground. Colonial past, using other nations' resources, anyone? IMO, it never would have washed.
South Africa's not a realm so there's no post of GG for Harry to take. And if there was it wouldn't be available for him anyway. It would be for a South African person.
I doubted it when it first came out that it was viable (this was even before the tour was announced) but there seems to have been a legitimate discussion about it, with a couple of sources talking about it almost happening then not at the last minute - after the tour they decided it wasn't possible. And it is one the the places they themselves mentioned in the Interview for whatever that is worth. Jacinda Arden seems to have known there were a couple of informal feelers out for NZ although she said nothing was ever discussed with her.
I assume they'd have lived there as "private citizens" like Harry did his gap year and they'd do mostly off camera charity and conservation work. There might certainly be problems with that both with image and cost (especially if they kept flying in and out) and maybe its one of the reasons it didn't happen but I don't think it was entirely impossible. Harry wouldn't have been the only rich, white even titled European to live and work there and he even has maternal cousins in Cape Town but that lifestyle doesn't seem to have been what they had in mind at all.