We've done JNB - LHR on BA four times and haven't had any problems getting J rewards - it's the obvious one to go for but unfortunately BA now doesn't release rewards predicably at 354 days out like they used to. I think you are still likely to get reward seats (Y or J) for that route but my guess is that you will have to wait till about 60 - 90 days out. Suggest you do some dummy searches to get a feel for when availability might show up. The taxes are steep but, as you have found, there are few options.
I suggest you forget about CPT - LHR because that's a popular route for British holidaymakers heading to Cape Town. I always found availability on that route poor (one year we stayed in CPT and could only get availability to LHR by flying back through JNB).
Suggest you also dismiss QR through Doha as an option. Some say QR now releases rare availability to QF only at 3 days defore travel, which makes it an unrealistic options for OW reward trips.
WY is the interesting one. I did spend a bit of time last year searching WY reward options when it was announced they were joining OW. Back then rewards seemed to be regularly available on routes they fly daily (such as MCT - LHR) but where they only fly to places 3 - 4 times per week (such as ZNZ - MCT) I didn't see much availability at all. Even though you can't book their flights as part of a OW Classic Reward yet, QF is still showing rewards availability for later in the year now, which helps with planning ahead.
Apart from the huge amount of extra miles burned in flying back to SYD, the obvious problem that counts that out is that under the OW rules you can only depart the country of origin once.
FWIW that LATAM route from JNB to Sth America you mentioned was always a dud. I searched for rewards on that route frequently for several years and never once did I see even a Y reward available.