Several landscape photographers on YouTube have dropped their D850 for a Z7.
Reasons mostly deal with a lighter weight camera though some prefer (and some don't) the EVF.
The biggest issue I would have with switching would be the cost of course but also the current scarcity of native Z mount primes. Using an adapter is a kludge I'd avoid
In general, for landscape I find I prefer EVFs when using heavy ND filtration (OVFs are worthless when using 10 stop plus ND filters), but OVFs when using polarizers or grad NDs. DR of sensors is so good now a days though that there's really no reason to use grad NDs anymore for most of the typical use cases, and one can just expose for the highlights and develop for the shadows. I'm also rarely shooting in light these days where a polarizer makes a lick of a difference (except for getting rid of reflection on water surfaces).
The weight change is a huge deal. With my MF kit, I went from a H4D-40 to 645z to X1D, and the smaller size/weight of the X1D is a game changer. Supposedly it's also easier to design smaller higher quality lenses for mirrorless than dSLR-style cameras. I've been using mirrorless 35mm cams for awhile now (A7R/A7RII/X-E3/X-T1), and forget how easy it is to hand hold all day and how portable they are. Last time I shot a wedding, I rented a D800 and 70-200 2.8 and my arms were sore for days afterwards. Not so much with a mirrorless setup. I just picked up a used D800 for my wife because she missed shooting with a dSLR. I missed it too out of nostalgia, but after playing around with it more, I think my preference at the end of the day is mirrorless. Another advantage of mirrorless is on-sensor AF. The Z7 has way more AF points distributed across the entire sensor than the D800 (and probably D850).
If I were you, I'd just stick with what you've got. You seem to enjoy the D850 and there's not much sense in selling it and taking a loss to switch to a system that doesn't have a complete native lens lineup yet which you desire.
Back to the Z7, I liked what I saw in the Z7, but I'm waiting until they've got more lenses out too. I'm already playing the "what is the pricing/availability of roadmapped lenses, and when will they actually get in my hands" game regarding lenses I want/need for the X1D, and have zero desire to do the same with another system. Happy to keep using the Fuji X-system and my wife's D800 for now, at least until the 24-70 2.8 and 70-200 2.8 are out and in stock at retailers.
