As others have said, with a week definitely don't waste time overnighting in Joburg on arrival. Book a through connection to Cape Town, unwind and enjoy the city for 4 days, then take the non-stop service to Skukuza (SZK) enjoy your 3 nights at Lion Sands and then the SZK-JNB flight and head home. Skukuza airport is actually partially run by the same family that owns Lion Sands, and you would board your 4x4 open vehicles at the airport for a short safari transfer to Lion Sands directly from the airport.