If you like your 810, stick with it. Best to go with familiar equipment you know rather than get new and then learn in the middle of your trip or worse yet, not know how to unblock....
24-70 or 24-120 on the 810 is good along with the 70-200 as your basics. Nikon also makes a nice 200-500 5.6 zoom, but it grows as you zoom.
When I rode the ferries up the Marine Highway years ago, 400-500 seemed good for the distant wildlife (300 mm 2.8 with TC 14 & TC 20). Today with better digital the 300 f/4 PF would be my choice with a TC 14 or TC 17 and my long lens needs are pretty fulfilled with a stabilized, small and light prime.
Be forewarned, a lot of equipment from Nikon is on perpetual back-order and shipping can come to a crawl...
If you do go with new, the D850 is a super replacement for the 810 and your 810 can be your secondary body or the Z6/Z7 with the 24-70, 70-200 and the new matched TCs is quite nice. If you can get them sooner than later.
Between the Z6 and the Z7, if you are not making large prints, the 24mp of the Z6/II and it's a little better lowlight is great. If you are going to "sofa-size" prints or don't want to pack long glass (400mm and longer), go for the Z7/II and simply crop. The crop mode on both Z7 and the D850 gives you a pretty nice 20MP image, like a "free" 1.5x teleconverter ;-)
Any change to newer equipment before travel needs to be done weeks before to make sure you find no issues, have the time to test and practice the new tools and figure out the idiosyncrasies such as battery life, best settings, color/iso settings, create new muscle memory. Also, don't forget about the hidden tax of new flash media (The 850 takes both SD and XQD, the Z6/7 take XQD but can be updated to C-Fast and the II of either also takes SD cards. SD cards in a dual-card system are slower and most of the cameras will default to the slower of the two cards installed.) Also, make sure your software and computer system can handle the newer raw formats and you have plenty of HD storage...
In any case, the main thing is to have a safe, trouble-free (especially with your photo gear), fun journey and you'll be in a spectacular environment with quickly-changing weather that can challenge even the best and that's part of the wondrous journey that you will remember for ages.