I know you are looking to rent a second camera, but, have you considered buying a used one. The reason I bring this up is, the 80D, (a great camera, I have one), being a crop sensor camera, doesn't really give you more reach over a full frame camera, it's just a crop of what you'd get from your full frame.
Renting an 80D is certainly the least expensive route at around $200 for the three weeks but a trip to the African continent with almost endless photo opertunities pretty much demands the best gear you can bring.
Here is what I'm talking about. A used 5D mk II would cost almost as much to rent for 3 weeks as it would cost to buy one. The reason I suggest the 5D II is, it is similar in megapixel count to the 80D, 21 vs 24 but the pixels in the 5D II are almost twice the size as those in the 80D. With a good size telephoto zoom (or prime) and cropping, in theory the image quality of the photos from the 5D II would be much better than the 80D.
The 5D II is a very sturdy, well made camera that can definitely stand up to rugged use. It's also pretty good at video. You already have a mk IV so you're familiar with the camera ergonomics and menus and decent used ones aren't expensive, and it uses the same battery as your mk IV.
Just a suggestion...

I know you been told this but it’s wrong a 70mm to 300mm L is still 70to300mm the way the lens work on the smaller sensor it’s focused on center of the sensor it looks like it bigger it not remember what I pointed out first FOV field of view. You can go google and see this and compare sensors what’s happening. You certainly are not going to to get more magifacation out of 70-300mm lens.

Try this site see if this makes sense to you see below
https://en.m.wikipedia.org/wiki/Image_sensor_format

I understand what you are saying, and that is part of the reason I suggested what I did. First and most importantly, you need to understand that the crop sensor does not give your 70-300 any extra reach. If you take a photograph with your full frame camera and a photograph with an 80D, same focal length, same location, then crop away the portion of the full frame image that is not in the crop frame camera image, the images will pretty much be the same.
A smaller sensor does not magnify the image, it simply reduces that amount of area there is to record with.
That is why I suggested the 5D mk II. With it's full frame sensor, 21 megapixel resolution and large pixels, a cropped image will have better IQ than what the 80D is capable of.
I also have the EF 70-300L, nice lens. I don't use it as much as I use to since I got the EF 100-400L II. One of the biggest advantages the 100-400 has over the 70-300, other than focal length, is how well the EF 1.4X teleconverter works with it. If at all possible, you may want to consider selling or trading in the 70-300 and picking up a 100-400L II and if at all possible, the EF 1.4X III teleconverter.
Best of luck with whatever you decide and enjoy your trip.