I would absolutely not try this. SA immigration can be insanely slow and although I do not check bags myself, I wouldn't imagine that baggage claim is any quicker. And then you get into the often massive queues for check-in at the domestic terminal. Beyond all that, if you have not transited JNB before, you may find it quite confusing--it's a big airport, with a domestic and an international terminal separated by a substantial walk, as well as the added excitement of some international flights going from the domestic side, among other complications.

Change the JNB-PLZ flight to later. Why did you book this itinerary as two separate tickets? You could have done the whole thing on BA (which has a domestic SA subsidiary)--they have multiple flights a day on that route.