The Zulus had plenty of firearms OTL, what they lacked was a doctrine to use them - and capturing more at Isandhlwana didn't change that and wouldn't in any plausible ATL. Furthermore, any strategy that relies on repeating Isandhlwana is only likely to deliver a Gigindlovu or Ulundi instead - that combination of luck (for the Zulus) and incompetence (for the British) is not something you can rely on happening again.
How about an earlier POD, such as a different Blood River? If the Zulus can avoid a direct assault on a fortified position and instead annihilate Pretorius' commando whilst it's on the move or something, then all sorts of potentially interesting things could happen - no Republic of Natalia (thereby forcing the Boers north and away from the Zulus), no British in Natal (Durban was occupied to deny the Boers a port), possibly even a successful invasion of Swaziland, by which point Zululand is starting to look seriously large...