I've been travelling LHR ~ JNB or direct to CPT for 49 years. It is MUCH smoother than it used to be because weather rader is GOOD
That said, there is something called the
Intertropical Convergence Zone (
ITCZ)
https://en.wikipedia.org/wiki/Intert...nvergence_Zone and that is unpredictable. Sometimes, turbulence cannot be avoided. On my last trip of LHR/JNB/LHR last November it was as smooth as I have become used to.
SAA still have a good reputation and no worries, both domestic and international. Do let us know how you get on.