So the Zulu were pretty much revolutionary in tactics and warfare among the tribes of southern Africa, and even with iron-age weapons they still managed to beat the British and scatter the Boers a couple times.
Now how could they upgrade their military tactics so that their armies would be more "modernized"? Maybe have a special regiment of musketmen, or hire Coloured mercenaries to counterbalance the infantry? Or hire Portuguese teachers(from Mozambique) to educate the Zulu in operating artillery, or maybe get their hands on enough horses to build a good cavalry? Or just for badass points, they could breed horses with zebras/quaggas and ride zorses into battle; apparently they're hardy and powerful and they aren't as temperamental as zebras.
In my TL the Zulu are supposed to survive, but to do so they're going to need a better military so that they can fight the Boers/ British and actually hold their own. I might use some of the ideas posted on here, and mix them with my own ideas for the TL so that I can write something realistic.