Just my two cents, as I love traveling and even more so, traveling light.
I’ve owned both the D810 and the D750 and prefer the D750 for traveling (and for events as well). It is lighter and performs better in low light. A3 Prints look fabulous. It might be nice to have the extra megapixels if you love/need cropping.
Regarding lenses. I try to challenge myself to take as less lenses as possible. I do the same for events. Personally I prefer prime lenses and I have shot events with a 24, 58 and 300mm (or 135mm) lens and did not say once ‘ I wish I’d taken... During my last 5-week trip to Australia and Tasmania, I brought the 24mm 1.8 (especially for the Sydney fireworks), the 35mm 1.4 Sigma (I would have brought the 1.8 if I had one), the 60mm 2.8 macro and the 135mm Zeiss (yes, that is quite heavy_. However, when I went out, I took max. 2 lenses with me. The 35mm and 135mm lenses were used 90% of the time.
My problem is that before I leave I always think that I need to take more lenses with me... I always feel the need to cover every single focal length “just in case”. In reality it is never necessary. So I try to bring one wide-angle, one normal to light Tele, and one Tele. I learned to zoom with my feet (after purchasing the Z6/24-70 kit, I also zoomed with my foot instead of the zoom ring).
To me, a D750 with 3 1.8/2.0/4 prime lenses is a perfect set. I try to follow Birna’s '2-3x difference in focal length' rule as it works well for me. I am thinking now about only taking a 35mm lens (ok, and perhaps a 135mm lens) with me.
In your case, I do not really see the added value (for travel) of a D810 to the beautiful camera you have unless the 2mm extra width (16mm instead of 18mm) is worth the extra cost and weight. And unless you want the camera anyway ;-) Perhaps you can get a loan D810 and compare the two side-by-side instorting things you like to shoot?