Looked at ex-JNB vs ex-Tokyo, similar pricing DONEx around 7500
Yes, JNB is only about $300 USD cheaper than NRT. Depending on where you want to go, ex-JNB/CPT usually means an extra continent, so there's that to consider.
Regarding ex-JNB or ex-Africa routings - is there any way to go eastwards on the RTW? having difficulty finding routings on the webtool where the last leg is USA-JNB, ie JNB-europe-asia-USA-JNB.
You can start by flying JNB-HKG, JNB-DOH, or JNB-SYD. You can't finish by going U.S. to JNB in one segment, though. You'll have to connect in Europe or South America.