What are you going to do with the pictures you take?
As strictly an amateur, I occasionally print 4*6 on inkjet. I usually display pix on a 27 inch UHD monitor, on a 40 inch HD TV, or a 65 inch UHD TV.
I considered getting a camera with replaceable lenses. But I bought a NIKON p900 which zooms to 83X. It does not take location data from the cell phone even though it has GPS & WiFi. For BIF though, you will not be doing that indoors and will be outside long enough for the GPS to work.

I did not verify this but I was told that the p900 weighed less than the equivalent 2000mm lens.

Simple answer, no, not for shooting BIF with a good telephoto lens. I shoot the Sony a9 with the 200-600 mm Sony, so no weight advantage for using a decent telephoto for BIF.
1. You could purchase a Nikon 500 mm 5.6 PF lens, it is light and very easy to hand hold.
2. There is no Z8 in the future, no Nikon announcement, no Nikon Rumors to solidly support that.
3. The D850 paired with the Nikon 500 PF is a dynamite combination and will serve you well for BIF, and if you ever go Nikon mirrorless in the future, just get an adapter and put that 500 PF on your fictitious Z8.
Good luck and keep on shooting until the end.

Looking for advise from all the Hogs. Current equipment base is a Nikon D750 and Nikon D500. Lots of lens ranges to accompany. As I age, I find it harder and harder to shoot BIF for long periods of time with longer lenses. I have used a gimble but am not totally happy with that solution. Am now thinking of changing to a Nikon D850 or a Nikon Z series (probably a Z8). Is there a significant weight tradeoff by going mirrorless and am I trading other benefits by moving from DSLR to Mirrorless format. I know that is the future direction (Nikon just discontinued the D500) but wonder if the 850 will give me more benefits than the Z series. Am not doing any video work, just still imagery.

The OMD (formerly Olympus) and Panasonic use a common mount that was developed some time ago when they formed the 4/3 alliance. However, you get the best results for image stabilization with OMD lenses on OMD cameras and Panasonic lenses on Panasonic bodies. From what I have read, the two companies implement lens stabilization differently so when the lens manufacturer is different from the body manufacturer, you don't get the full range of stabilization possible with the two working in conjunction. I had a Lumix 100-300mm on my E-M1ii and had to use either lens or body stabilization, but not both. When the lens and body manufacturer are the same, the stabilization is much improved (SyncIS on OMD and Dual I.S. on Panasonic).

There is no question that the manufacturers have decided that future equipment will be built to the mirrorless design. There is also no question that there are some benefits provided by that design. My personal belief is that benefits claimed in the area of weight and size have been exaggerated.

Your two DSLRs have different user interfaces. I don't know if that is a big deal for you or not. The Z6 and Z7 have interfaces like your D750. The Z9 interface is like your D500, as is that of the D850. Shooting the D850 is exactly like shooting the D500, but the camera is a little heavier and slightly larger.

I have just started exploring IR photography using a converted D90 that I just purchased. If this develops into a serious pursuit for me, I'll be looking toward a Z8 conversion, provided that it is ever actually introduced and that it has the professional interface of the D500 and D850. The mirrorless design offers very concrete operational benefits in achieving focus while still using the viewfinder when doing IR photography.

No one can or should tell you what camera to buy to meet your specific needs. You will need to investigate for yourself whether any particular camera will address your shooting challenges.

Looking for advise from all the Hogs. Current equipment base is a Nikon D750 and Nikon D500. Lots of lens ranges to accompany. As I age, I find it harder and harder to shoot BIF for long periods of time with longer lenses. I have used a gimble but am not totally happy with that solution. Am now thinking of changing to a Nikon D850 or a Nikon Z series (probably a Z8). Is there a significant weight tradeoff by going mirrorless and am I trading other benefits by moving from DSLR to Mirrorless format. I know that is the future direction (Nikon just discontinued the D500) but wonder if the 850 will give me more benefits than the Z series. Am not doing any video work, just still imagery.