Advice needed:
I will be doing some long term travel - with a lot of trekking - in 2018, and will likely pick up a D850 for this trip.
I think you may need to give us a bit more info on what trekking means to you. I would think it means you carry some food, some clothing, sleeping bag, probably a lot of water, and your camera gear. Will you be carrying a tent also? Are you trekking in the mountains or mostly level terrain? Are you very fit (= running marathons and stuff)?
My experience from multi-day hiking in the mountains (> 10'000 ft) is that it totally kills me, and that's without any heavy glass. I often bring a 18-35 + 70-300 + D610 + tripod combo, sometimes with the 20/1.8 on top (don't know if you are interested in this, but most of what's shown here is shot with this kind of setup:
https://500px.com/basilgreber ). The two wides combined weigh less than the ultra wide zooms you are considering. The D610 weighs less than the D850. And so on. I fully understand you want the best gear, but at some point you will also have to ask yourself (or rather - tell us, because you probably thought about it) about the maximum load you can carry. A 50/1.8 or 50/1.4 is not very sexy, but it gets those shallow DoF and low light jobs done reasonably well. And weighs much less than a 24-70/2.8 E. Oh my.
From my own experience I can tell you that I take better photos when feeling reasonably well rather than completely exhausted from a massive load of gear.
In any case. Good luck with it all.