Where in Africa are you going? Depending on the location and type of reserve (public vs private) there might be some differences in how you build your kit out (and how you potentially do or don’t spend money). For example, the ability to drive off-road can greatly impact how near to animals you can get. Other places have some pretty epic animal-scape opportunities. Some other questions it might be helpful to know the answer to in order to get the best advice on here are:
1. Are you open to renting?
2. If you aren’t open to renting, as Calson mentioned, what else do you typically enjoy photographing?
3. Is there any sort of weight restriction for your travels/are there any packing considerations that might impact what you take?
4. Are you planning on just one body and one lens? Or two bodies and multiple lenses? Renting can also help with this, and if taking meaningful pictures is important to you then some sort of backup strategy is probably worth considering.
5. Are you planning on going on safari ever again? Or is this a once in a lifetime trip?
With all that being said, I think if it is a once in a lifetime trip, and you are doing one body and one lens I’d look at renting or buying either the native Z Nikon super zoom or Tamron superzoom. If you are buying, the resale value of z mount glass will hold up better than buying f mount at this point (although maybe you could find f mount glass cheap). The Tamron is readily available and I’ve started seeing the Nikon show up in some brick and mortar shops (albeit briefly). If you are open to taking more than one body, I’d probably suggest either of those super zooms on one body, the 24-120 on a second body, and take the Tamron 100-400 you already own as a backup.
Another reference you may want to look at is Thom Hogan’s website, as he safaris in Botswana almost annually. Here is a link to a fairly recent article that talks about various kits with Nikon Z glass, and you could easily substitute the Tamron 150-500 for the Nikon 180-600.
https://www.zsystemuser.com/nikon-z-system-news-and/the-second-safari-lens.html