Why not start in Johannesburg which is currently one of the least expensive places to start a OWE?
You could do:
JNB-SYD-PER-NRT-DEL-NRT-SIN-HKG-JFK-DFW-ANC-DFW-YVR-LHR-MCT-LHR-JNB (or CPT) which I think covers everything you wanted to see, and is 61022 miles.
It's a DONE5 which is 45300 Rand or about £3120
Of course you have to get to/from JNB/CPT before and after, but I don't think that will be any harder than MCT or MRU which you were planning to use before.

Not sure if you are also aware that if you purchase this before June 1st, you can still have 20 segments, so that gives you even more possibilities.

Happy Travels