First, your old primes are going to work well on the Zs - it's the best camera for old primes since you have focus peaking and magnification options in the viewfinder - manual primes have never been easier to focus. Plus, you'll have some degree of in-body image stabilization with them as well (not the full 5 axis, but you'll still have some stabilization).
As to the other challenges, it's tough. None of your choices are bad, although I don't know that I would want to use a D750 over a D850 - there's less than a 0.4 pound difference in weight and significant differences in technology. I don't know that I'd go that route.
The D780 is interesting though - it would give you the Z6 sensor and better low light capabilities than you have with the D850. Video is outstanding too. The AF system isn't as good as the D850, but it doesn't seem like you really need those capabilities. Still though, the weight savings over the D850 isn't huge (about what you'd have with the D750).
I think if weight is the primary concern, one of the Z cameras are really the best choice here.
Another thought is to get a first generation Z camera. They use the same sensors as the II versions, so no loss in IQ. Their performance capabilities are more than adequate for landscape work and the price is far easier to swallow. Heck, you can probably land a used one pretty soon for less than $1000. Just a thought.
Your advice is excellent, Steve... practical and Informative !!