Torn to freaking shreds. The white minority would use chemical weapons with abandon, and possibly tactical nukes on the Bantustans. The casualties would be the worst since WW2. And outside powers would be very wary of intervention due to their troops being caught in the middle of that.
The white minority would see it's very survival as at stake and would fight without reservation or holding anything back.
The black majority would fight equally if not harder.
I don’t think the casualties would have been that high as neither side had the capacity to sustain a full scale war for very long. For the SADF, facing a global embargo, every piece of large military hardware was precious. Some, such as Tanks and aircraft were irreplaceable. Meanwhile, for black rebels, every weapon, piece of equipment and munition would have to be brought in through a road network controlled by well trained, motivated and very mobile SADF units.
I think for the rebels to do well, they needed to inflict equipment and human losses on the SADF. Whites were not only a minority, but they really were not accustomed to casualties. This would mean moving the fighting out of the open bush where the expert SADF was used Israeli / Afrika Corps mobile warfare tactics to win entire campaigns with very few deaths. Instead, the rebels would have to force battles in urban areas or in rural areas with restrictive terrain that forced SADF units to dismount and then fight continuous, casualty producing and equipment destroying battles.
Even if rebels could do this on a large scale, I don’t think they could truly win the war. Mobile SADF units would defeat every rebel unit that ventured into open terrain. Meanwhile SADF units would be increasingly reluctant to venture into rebel controlled areas. In short, I think the SA economy would crash and the war would widen internal fissures in both black and white societies.
There would be no Armageddon. Rather, a continuous small scale war between two general sides devolving into factions similar to Lebanon or Bosnia. Both general sides would field a mixture of main force units and increasingly rouge irregular style units. On again, off again fighting, ethnic cleansing, the establishment of safe areas, liberated areas, go and no go zones, massacre and counter massacre, local peace treaties, temporary alliances and double crosses would be the norm until the factions got sick of it and made peace with each other.