Congrats on the gear! I look forward to hearing how the lens performs. As David mentioned earlier, it could be a lightweight solution for me, eventually.
Cool...congrats! What part of Africa?
As for Iceland, I went in 2019, using my 5DIV, of my keepers, I was 7% EF 16-35 f/4 L IS, 82% EF 24-70 II, and 11% 100-400 II. I went in March. If I were to go in summer, I would also be targeting puffins (not present in March) and would make sure I had longer lenses....but, your 100-500 w/ TC might be all you need. I also had rented a Sigma 14 f/1.4 to photograph auroras....but we were clouded over the entire time so it was never used. In the summer that likely would be a non-factor due to the lack of darkness.
As for Africa, absolutely, yes, I would have a second body with me. Depending on where you are, I often hear that dust is a significant issue. Bring plenty of wipes. But, due to the dust and wanting to respond to a scene quickly, most people I see have 2+ bodies with lenses mounted. Which lenses depends a bit on where you are going and what you are targeting. Birds, always need reach. But large mammals, when I envision Africa, I am thinking large mammals in their environment. Thus, it isn't reach that you always need. But I do often hear about needing light. So, as I envision an Africa trip, I could see something for reach, your 100-500 may cover that, but then 100-300 f/2.8 with or w/o TC? 70-200 f/2.8? 24-70 f/2.8 and I've even heard some people talking about taking the 28-70 f/2.
If I was going today, I'd likely do RF 24-105 f/2.8, RF 100-300 f/2.8, and 500 f/4 II with the RF 16 f/2.8 included if I wanted UWA. I'd have at least 2 bodies and would consider 3. But when I think of that, I am in a jeep on the Serengeti with a whole row to myself.
Again, depends on where you are going (Congo? Serengeti? Botswana? S. Africa?) and what you are targeting.