I'm from Cape Town (not read the full thread yet), but re weather: it may or may well not be cold and rainy in August. It probably won't be beach weather, but you might well have warmish (18 - 20C) sunny days (or you could well have storms).
Re safety: if you stay in the touristy or wealthier areas, you should be fine as long as you take the precautions you would in any large city (keep car doors locked, use your safe, don't flash or leave any valuables unattended). Ubers are widely available.
Cape Town is much safer than Joburg.
If you want hot (30C) beach weather, December to Feb are the best times.
I would recommend taking a day or overnight trip to Franschhoek (beautiful historic wine estates) and Hermanus/Betty's Bay (whale watching between June and Dec as well as a penguin colony in BB.
In CT I would strongly recommend Boulders Penguin Colony and Cape Point as well as Groot Constantia Wine Estate, the whole Muizenberg to Simonstown area, the Waterfront (if you like art then check out the Zeitz Mokaa museum of Contemporary African Art), and the Bo Kaap.
I would avoid the city centre - it's pretty grotty and not v. safe.
There will likely be scheduled power cuts of around 2 hours duration.