At least 90 minutes is legal and you're on one ticket with SA all the way. So if anything goes wrong you know SA will look after you.
90 minutes is quite doable provided nothing goes too badly wrong. That would include late arrival or, as happened to me recently, 90 minutes from wheels down to bags on carousel. (And that was at 8 in the morning. I think there is a lot more pressure in the mornings with all the flights arriving from Europe and Asia.) Add to that the fact that the 19:30 is SA's last departure for CPT on a Saturday. So if things do go pear shaped SA would presumably put you up in a hotel and fly you out next day.
So the question you need to answer is: how much of a disruption to your plans would that be compared to the alternative of booking yourself into an airport hotel and flying the next day anyway?