I traveled and safaried Africa for 3 months, South africa, Namibia, Botswana, Zambia, Milawi, Tanzania, Kenya, Uganda. I took 7DMKII, 5DMKIII 100-400mm MKII, 24-105, 16-35 and 1.4x.
TBH unless dusk or sunset the 5DMKIII had a 24-105 attached. I must have a good copy of the 7DMKII because mine never missed a beat and was my go to for wildlife. I was happy with it up to 4000 iso and at times did hit 6400 with a bit of work in post.
AF was super fast. The other thing to bare in mind is past 500mm you tend to get atmospheric issues so I was already hitting that with the 7DMKII and 100-400 combo at 640mm. TBH the only thing that would have been nice would have been F4 or larger but unless its a 400DO or 300 F2.8 the larger lenses are incredibly difficult to work with especially shooting on safari with a few people in the vehicle. Unless its your own vehicle or a specific photographic tour vehicle, when the canvas roof is up moving the lens around in the 6ft space to capture moving animals is a nightmare. People that I was with using them werent that happy either they ended up having another 2 bodies with a 100-400 and a standard zoom. So for me the 500mm just wasnt the right lens. The nice thing about the 100-400 is its size and the zoom. Its IQ is stellar its a great compromise.
I was backpacking, I also took an 11" MB air and 2-3 drives so my photographic gear bag was already 15kg with the above gear not including water and daily essentials then I had a 15-20kg bag with my clothes etc so a big white wouldnt have been feasible.
The nice thing about Africa is travel is easy so I didnt carry the bags oftern, and I also camped for 90% of the trip keeping the gear safe was a concern.
I also trecked a lot like Biwindi national park in Uganda, about 40 deg ridiculously humid and I trecked for 6-7 hours through the rain forest cutting a trail with machetes with my 7DMKII 5DMKIII 24-105 and 100-400 with food and water. I was 26 and pretty fit and it was hard work but was an incredible experience.
Not sure if thats a concern or you are generally in the vehicles a lot?
I found that the 1.4x got used once or twice and I wasnt overly impressed with the combo on my 7DMKII but on the 5DMKIII it was useable but found AF slow and IQ better on the 7DMKII bare.
Also there was only a couple of occasions where I needed more than the 100-400 some birding and a leopard in a tree. Other than that you get so close its crazy, many worry about what they need for safari taking big whites but in 90% of cases its not needed.
For example this pride just followed our defender
they were so close I was using the 24-105 most of the time and the 100-400 for portraits.
The 5DMKVI would be a great upgrade and thats my next body but im going to keep the 7DMKII because I really enjoy using it and its speed is fantastic. 1DX would be nice but I think 20mp is a little too limited for all aspects like birding etc for large mammals it would be no problem. You would have to take a big white.
For me with the heat and the miles I traveled which was around 20,000 I needed to think about my own well being as well as my photographic goals. Some of the people I saw trying to wield these lenses was comical and im sure a lot would have been better off with gear suitable for them. It was a compromise for me, not just the images but my back. I think the compromise was a really good one.
The first big travel I did was 2 months across south america and I went to the amazon as well as Colca Canyon etc I took my 5DMKIII 24-105 16-35 and 70-300 L and the kit was fantastic so light in comparison but 300mm on a FF body was so short I came away very disappointed in the Amazon. Although I got some great shots I was cropping more than 50% so wish I had taken a bigger lens.
That said there is no in between lens between the 70-300mm and 100-400. The 70-300 is so compact it can sit upright in a bag like a 24-105 and is less than a KG the 100-400 is 2-3x the size 50% more weight. For a backpacker its a tough one. The second time round I bought the 150-600 tammy wasn't impressed so got the 100-400 with the 1.4x to ensure I would have enough.
The other option I would look as is a 400 DO on a FF body, standard lens on another and a 100-400 on a crop. The 400DO is probably the best combination. The 300 would be a great choice too but I think I would end up having a 1.4 attached to it all the time so would take the 400.