I did a trip to Botswana this April & May. The first piece was in the Kalahari and included Makgadikgadi Pans National Park, Dinaka Reserve and Nxai Pan National Park. The second piece was in the Okavango Delta and included the Moremi Game Reserve, Camp Okavango, Chobe National Park (Savuti) and the Khwai River. Great trips.
I took a Z9 and Z8 for bodies. I had the Z 400 mm TC, Z 100-400 mm and Z 24-120 mm for lenses. I used my iPhone 14 Pro for camp shots. About 2/3s of my photos were taken with the Z 400 mm TC -- a few more with the internal TC engaged (560 mm f4) than with the bare lens (400 mm F2.8). About 1/3 of my photos were taken with the Z 100-400 mm lens, mostly when subjects were closer, bigger, or when I wanted more of the environment in the shot. I took a relatively small number of photos with the Z 24-120 mm, but I was glad to have it along. Both the Z 24-120 mm and Z 100-400 mm lenses have a good minimum focus distance for close ups of small things.
Given you have a Z8, another Z8 makes sense as a second body. You could consider renting one rather than buying one, depending on whether you would use 2 bodies after the trip. Two bodies are useful to avoid changing lenses in the field, where it is often dusty. It also gives you a backup if one body fails or is broken. The Z8 will also let you do a reasonable crop in camera or in post that can be helpful at times.
The Z 180-600 mm gives you a nice range of focal lengths. A number of people on my trip used that lens and were very happy with it. They tended to pair it with the Z 70-180 mm or the Z 24-120 mm. The Z 70-200 mm would also work well. You will get less background separation with the Z 180-600 than I got with my Z 400 mm TC, so you will want to pay attention to backgrounds.
If I were taking the Z 180-600 mm to Botswana, I'd probably pair it with the Z 24-120 and the Z 70-200, if I had space for it and was within any weight limits. (Weight and space would not have been an issue for this combo on the trips I did.) I'd also throw in a Z 2x TC that could be used with the Z 70-200, giving you a 140-400 mm f5.6 lens as a backup if the Z 180-600 were broken or failed.
Given the low light that you will likely encounter, you will want to have good noise reduction software. Adobe Camera Raw and Lightroom have good choices. Topaz and DxO can be useful as well in my experience.
I do tend to think about what I would do if a given piece of gear were broken or otherwise failed on a trip. Not that you need or want two of everything, but some functional overlap is good. I've seen gear broken or otherwise fail in some remote places (Galapagos Islands and the Antarctic Peninsula) where you can't easily get it fixed or replaced.
Good luck on your trip. Botswana is a wonderful place to visit.