The primary issue with LVI is that there is only one flight a day on Comair (it’s not operated by Kulula btw), I believe only one a day on SAA as well, which IIRC is before the BA one. So if you miss the “connection” you’re overnighting in JNB anyway
i did this exact “connection” on two separate tickets a few months back, and it was just fine. However, I was quite nervous and of course “I did it and it was fine” is no guarantee of success
you could check and see if any of the VFA flight give you any more time to connect- the KAZA univisa covers you for both countries and the transfer across the border is OK if a little complex, definitely an experience