I agree with Dave, good time of year. Might be a bit steamy in/around
Durban, but if your car has aircon you'll have an emergency escape from
the heat if you happen to need it.

I also agree that the drive should be Joburg to Kruger then down the
east coast south of Lesotho and west across the Garden Route to Cape
Town. I would add an excursion up the west coast to Namaqualand, which
is unforgettable and an amazing contrast to the lush tropics of Venda
(northern Limpopo) where you'll begin. It is safe and well sign-posted,
with plenty of petrol stations, accommodation, etc.

Fly back to Joburg -- buy a low-cost one-way on Mango, Kulula or 1Time
airlines.

What kinds of things are you most interested in? There are great
archaeological sites all along the route, historical battlefields,
botanical wonders, hikes, fly fishing, animals, golf, music, spas, sea
and sand (I imagine you aren't keen on learning to surf, but one never
knows -- turning 60 tends to do things to one's mind...) and animals.
Are you foodies? Wine lovers? Vegan? You'll go through vibrant cities,
deep rural traditional areas, and will likely hear all 11 official
languages as you go - the cultural diversity is stunning.

Have you considered hiring an RV and taking your own bedroom and
kitchen with you? There are several excellent companies who offer
those.

I'm happy to make more specific recommendations of routes and
destinations (as others are too, I'm sure), but by and large this NG
has tiptoed around endorsing specific accommodations (B&B, guest house,
safari lodge, etc.) to keep it from being spammed by adverts and
marketing blather -- but we will warn you away from something if you
ask and we know it to be a problem.

A month is a good amount of time...but be prepared to plan a return
journey!

Kurt