South Africa and Pakistan are bad comparisons for an NPT member like Iran

At the time of its nuclear program, South Africa was a non-signatory of the NPT as were Pakistan, India and Israel. There is no objective basis to compare Iran, an NPT signatory that has not diverted any nuclear material to a weapons program so far to any of the non-NPT signatories. If you must describe two cases among the non-signatories, South Africa, Pakistan, India and Israel all actually developed weapons. In no case was the nuclear program successfully stopped against an NPT non-signatory.

South Africa built nuclear weapons while facing no effective pressure from the West because just as the West favors Israel today, essentially for racist reasons the West favored South Africa during the Apartheid era. DeKlerk made the decision that since the regime of white rule was ending, he preferred that the next regime not have nuclear weapons. This is no example for any regime that actually continues in place.

The case of South Africa might be analoguous to the US successfully installing a new Shah in Iran, who might, after that regime change, follow US orders to disarm his country to restore Israel's nuclear monopoly. The best analogy for South Africa's dismantling of its nuclear program would be that if Israel realizes it will be forced to accept the Palestinian refugees and realizes that it can not continue as a Jewish-majority state but will soon have a Muslim political majority, its leadership will likely make the same calculation DeKlerk did.

A better analogy for Iran would be Japan and Brazil, or Canada and Romania, or Argentina and Taiwan. North Korea actually developed most of its nuclear capability before signing the NPT but even it would be a better example for Iran's case than any of South Africa, Pakistan, India or Israel who during the relevant periods in their nuclear weapons histories were not NPT signatories.