From the literature I read in grad school, sanctions tend not to work. It wasn't the sanctions that ended Apartheid, but a political awakening among South African whites and the National Party in particular backing off from their hardline stances.
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I can't speak for the literature you read, but watching Apartheid fall
live, as a young man, there was continued direct evidence, from the horse's mouth, as it were that sanctions, which had utterly strangled the South African economy, hurting of course everyone, but whites had the lion's share of the economy anyway, and the shift in attitudes and leadership were the near-direct result of financial, economic and political pressure from the left. It was visibly working in that regard. I think there's a lot of arguments about how we intervened in the economy subsequently, but there's very little evidence to suggest the white nationalist old guard were swayed by a sudden moral awakening. Bankruptcies, spoiled crops, internal dissent and a drain of rich people fleeing the nation all pushed saner voices to the top.
In fact, the biggest philosophical gulf between Botha's hardline stance and De Klerk was that De Klerk was
economically liberal and in that sense, a pragmatist. He simply saw the writing on the wall - a complete collapse of the economy and social fabric or suffrage followed by slow normalization. He was almost certainly right, and Botha's preferred chosen path would have been terrifying to see played out.
The South African economy now is larger and stronger than it was then, but adjusted for inflation it should be doing better. However, the distribution of that wealth is, unsurprisingly VASTLY better and more humane than during Apartheid.
