Hi and welcome:
I have not been to Antarctica personally, but I am a member of the NZ Antarctic Society and it so happens we had a couple of talks from people who have photographed extensively down there. I will add to that my own experience in shooting above the Arctic Circle - which does not get as cold as Antarctica does in winter or the interior, but you are unlikely to be heading into either - I assume you are on a cruise so you will be on the water on very close to it. It won't be that cold - you can check with the cruise company as to the temperature ranges they are expecting and I am sure they have advice on photography gear and preparation.
Right now the continent is experiencing conditions much warmer than normal - it is part of the massive changes happening rapidly there, and every report suggest that our models are far too conservative for how fast and drastically the conditions there are changing. My definition of warm is inclusive of it being around freezing and can get below that in hours. You will want a dry bag - coming out of the cold to a warm ship will immediately create condensation to the camera, but a drybag that seals will let the camera acclimatize slowly and reduce the risk of in-camera and lens condensation. You also want lots of batteries, a couple of chargers. Keep spare batteries inside your jacket to maintain their life and do the same with the grip battery magazine between shoots when you are out in the open.
Leaving that aside, if you are heading that way before late January, you will likely have essentially total daylight so you will likely not need to go down to f/2.8 a lot - it will be bright: there is a lot of snow and ice there! In that respect, I would debate the need to take a heavy tripod - I would recommend something that is a bit of a cross between a monopod and tripod, the iFootage Cobra 2 C180 Monopod. You can use it as a monopod but it also has the ability to split off to a short mini tripod that will carry the weight of your camera and lens. That short tripod may be very hand for low shots of penguins etc. on the land, and the monopod might be handy on the water as well.
Much depends on what you are going to produce - you can crop the 45MP images of the R5 series a LOT, especially if you are not intending to produce massive prints of extremely high resolution. With that thought in mind, the 100-500 should be sufficient to allow you to crop down to the equivalent of using it in APS-C mode, which will give you 17MP and an equivalent FoV of 160-800mm, so I would suggest sticking with the 100-500L. I personally would not take an extender - you don't want to be taking stuff on and off outside and if you are trying to get a sudden shot of a long-distant whale or whatever, it will be gone by the time you have tried to add the extender or the reverse.
Some interesting reference:
Antarctic Photography Tips (the Ultimate Guide with Sample Photos) (photoworkout.com)
The Ultimate Guide to Landscape Photography in Antarctica | Iceland Photo Tours (iceland-photo-tours...
cheers, TREVOR
The mark of good photographer is less what they hold in their hand, it's more what they hold in their head;
"All the variety, all the charm, all the beauty of life is made up of light and shadow", Leo Tolstoy;
"Skill in photography is acquired by practice and not by purchase" Percy W. Harris
