For the OP,
Sorry if you have already taken this trip? Not sure when you were going.
If you have returned do let us know if you had a good trip with your baggage dilemma.
Since the Delta flight from the US tends to arrive in the evening, is it correct to assume you will overnight at a hotel in JNB before your ongoing flight on a bush plane?
If so, what I did was to put my empty duffel bag in a locked, checked bag, get to the hotel, then pack the soft duffel bag with just the items I needed for the safari. I was able to leave my hard case at the hotel since I was returning back to JNB and had booked a day room. We did a safari first, then returned to JNB to go on to Mauritius in June/July so there were many "dual climate" things I needed to bring for each distinct location.
Alternatively, you can also leave a hard checked bag in the secure left luggage area at JNB for a reasonable fee and then just retrieve it and "repack" the duffel and its contents into the hard bag.
To be fair, I flew in Business or First each time so I had a more generous bag allowance.
Agree in the opinions to NOT put cameras,laptops,ipads, binoculars, or any other valuable items in your checked bags while in transit.
So far I have been very lucky in that I have not had any bags pilfered or tampered with despite many transits through JNB.Hope my luck continues. The longer a layover, the higher the likelihood things can go missing out of your bags, so I would never check anything through JNB-just short check your bags to JNB and get them when you go to baggage claim.