First I'd like to say that this is a really complex question.

Was apartheid SA really a white utopia where all whites were guaranteed a job and nobody needed to lock their doors?

The answer to the first part is that they weren't guaranteed a job per se, but there was decreased competition because of the apartheid system. Even before formal apartheid, there was a "color bar" that limited how high in the structure of businesses blacks could advance. Whites benefited from apartheid in this regard but they were not explicitly guaranteed a job. As for locking their doors, it is true that Apartheid South Africa had lower crime rates than post-Apartheid South Africa. This is a natural outgrowth of a police state. Police states are often aimed at preventing political dissent, but they also tend to reduce other types of crime as well.

An additional benefit to Whites under Apartheid was that the price of labor and goods was heavily depressed because of the way Blacks were treated. This meant that someone could live at a relatively higher standard of living as a white person in South Africa than they could have in the US or the UK.

Or did whites suffer from an authoritarian, paternalistic regime

The answer to this is yes. They obviously didn't endure anywhere near the suffering of black South Africans, but they were also restricted in invasive and authoritarian ways. For example, there were laws limiting Blacks to what are essentially corollaries to Indian Reservations for native Africans without certain documentation. This meant that they couldn't work in the (relatively) more lucrative jobs in the cities without permission. Whites were also not allowed to hire them and could be fined heavily if they hired someone that was not allowed to work. I want to stress again that they punishment would not be as severe for Whites, but they still had little choice in the matter and the fines could be fairly substantial. Many of the laws targeting native Africans also limited the freedom of Whites in some regard (though, and I stress this, not to the same degree).

They were also restricted by the authoritarian government.

As for the economic stagnation, a little quick research shows the GDP growing fairly steadily over time. I don't see anything that indicates that the government caused the economy to stagnate though it is impossible to know what would have happened in the absence of the Apartheid government.

tl;dr: Yes, white South Africans benefited from Apartheid, but they were also limited by the government politically and economically, though to a far lesser degree than Blacks. Basically, it's complicated.