What are you going to photograph? How far are you going to cary the equipment. Will it be backpacked in for a few miles or a few hundred yards? You are going to need a tripod with a 400mm lens, no way around it. So one important thing is to figure how much weight you can carry, don't forget you need a few water bottles too
I assume this is for wildlife photography. Bring a folding chair and a big hat at this will entail hours of waiting.
But what worries me is the words "A few years out" which implies to me you don't spend time inthese places regularly and won't for some time. Then you think the brand of lens matters. No. your experience matters a lot more. You almost certainly WILL be using a short lens more then a long lens and mobility and light weight matter a LOT.
I just got back from Yosemite. I'm there several times a year. This time I decided to take ONLY my 35mm f/1.8 lens and leave everything else at home. I use a DX format Nikon. Of course I also took my iPhone (kept in "airplane mode to preseve battery life as I had no way to charge it and zero "bars") to use mostly as a camera The 35mm was about right. I missed only a few shots I wanted. In your case with a FX body take the Nikon 50mm f/1.4
I hate to say it but the #1 beginner mistake is to think you need a huge lens. Mostly these long shots look poor because oil all the atmospheric haze you are shooting through. Some times you get lucky and the air is very clear. THEN is the time to break out the big lens.
One idea is to bring a macro lens. When you start learning how the eco systems of these places work, it all starts with tiny things
The BEST idea is not to wait. If "photography" is something you only do when you are far from home you will NEVER get good at it. A good plan is to pretend you are a tourist visiting the areas where you live. Learn to shoot travel shots that way. Shoot a minimum of 20 frames a week, every week and pic the best 2 or every ten. Give these the full post processing treatment and after a year you will have 100 good portfolio shots.
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Here is what I find: You will get the SAME number of good shots no matter what lens you put on your camera. But you will get different shots depending on what lens you have.
There are always more possible photographic shots then you can possibly do. Even with a personal assistant and a ton of gear you'd still have to decide NOT to shoot most everything you see and given that you have finite time you have to choose. So if I have only a "normal" lens (35mm for crop frame body, 50mm for full frame) then I will select about 20 or 40 subjects that I think will look good from the "normal" perspective. If I had only a 60mm macro lens I'd shoot the same 20 to 40 frames but they'd be of tiny objects. I find this happens up to about a 180 mm. It gets really hard, nearly impossible to find subjects that look best when shoot with along lens like that. I can do it but I have to do things like going to an isolated spot by a river at 5:00am in the morning before anyone else is up. It is about the ONLY time you see most wildlife in a crowded national park. But even then 200mm is to long, I want some context in the photos otherwise I'd go to the zoo and get Animal head shots.
So on my last trip (to a national park) a couple weeks ago I took only the 35mm f/1.8 lens and was determined to shoot every shot at f/1.8 I wanted a record of the trip where the images all have the same "feeling" and it mostly worked.
So try it: just ONE PRIME LENS for an entire week or an entire trip. I'd start with the "normal" lens for your camera. I guarantee you will get just as many good photos
