Perhaps the best bet for the Zulu (and the Xhosa, for that matter) is for the VOC to realize that they have a large population of potential farmers here and fairly early in the 18th Century, establish a trading base at Durban and /or Maputo, on Delagoa Bay, trading initially for slaves but soon realizing that a "culture system" in which the Zulu pay tribute in the form of crops the Dutch want, which is how the Dutch exploit Batavia, will work better. This kind of colonialism will do the least damage to the Zulu and Xhosa and will keep the Xhosa and trekboers out of each other's hair, hopefully, while securing a nice profit for the VOC, which is the entire point. Population will be able to grow and settled living will occur. Draft animals such as oxen o waterr buffaloes will be introduced and cities start to develop.
Or alternatively, when the Arabs from Muscat conquer East Africa in 1698, they go all the way down the coast, taking Mozambique from the Portuguese too and building an outpost on Durban Bay and another at East London. They may or may not take Capetown (probably not; Capetown has some pretty substantial defences). The Zulu and Xhosa develop as Muslim emirates.