what if instead of ending peacefully, Apartheid results in civil war between the South African goverment and the ANC
To return to the original post, South Africa was in a state of civil war by the late 1980s, but that war was irregular warfare
To get to conventional warfare, MK would need to be holding liberated zones, and attempting to expand from them. This was never in the ANC's political strategy, nor in MK's military resourcing and training
If the ANC was working towards that strategy, it would need much heavier military resourcing, and the USSR would have needed to be willing to supply this to MK, as they started supplying ZIPRA.
Then MK would need defendable bases in the border area. That presents a problem as most border areas were heavily patrolled by the police, and the apartheid army could be readily deployed there.
The one exception to that was the Namibia Angola border, where, yes, there was conventional warfare. The South Africa intervention / invasion in Angola was described to the domestic white audience as the "border war". It was largely conventional warfare and pitted the apartheid South African army, with a largely conscripted infantry, and their allies/puppets such as UNITA against the Angolan army, also with a largely conscripted infantry, with SWAPO and MK (whose training basis were in Angola and whose soldiers fought in this war), supported by Cuba and the USSR.
The end to that war and the South African military withdrawal from Angola*, lead to the independence of Nambia and paved the way for talk between the apartheid government and the ANC.
So in essence the large scale civil war you postulate took place, but it was fought in Angola.
*recognizing it was unwinnable or being defeated, depending who you listen to