From the DRC you might consider routing through Zambia and Botswana. There is not much avgas in the DRC, definitely none in Lubumbashi unless you arrange for some drums to be moved there. You should be able to obtain avgas at Ndola, Lusaka and Livingstone in Zambia. This is subject to the usual African shortages so check first with BP Zambia. Francistown and Gaborone in Botswana should have avgas. Alternatively you could fly non-stop from Zambia over Zimbabwe to Polokwane (Pietersburg) or Lanseria in South Africa and then on to your final destination in SA. All the places mentioned are international airports where you can clear Immigration and Customs.