Photography has so many tradeoffs between cost, weight/size, focal length, maximum aperture, AF capability, zoom vs prime, and sensor size.
The Canon and OM kits listed above look light weight and hence attractive. I've never shot them, so I can't comment on their performance. I've shot Nikon for many years and am currently shooting Nikon mirrorless with a number of telephoto lenses. I do mostly wildlife work, especially birds, and some landscape work.
Given my bird photography, I want the best AF I can get. (I'm sure I could also have great AF with other systems by Canon, Sony and OM.) I use a Z9 and Z8 for bodies these days. I previously had a Z7II and Z6II for mirrorless. My last DSLRs were a D850 and D500.
The Z9 is heavy and would not make sense given your desire for lower weight. The Z8 weighs about the same as a D850, so no weight savings. But it (as well as the Z9) has excellent AF for BIF and other action and in that respect is an upgrade from the D850. Sensor size and IQ for the Z8 (and Z9) are pretty much the same as the D850.
You could save weight by going to a Zf. It is about 10 ounces lighter than your D850. But it has only 24 mp, if that matters to you for printing, cropping and the like. I have held one at my dealer's, but have not used one yet. It sounds like AF is better than the Z7II/Z6II. Suspect it is not as good as the AF on the Z8/Z9.
You could also save weight using a Z7II. It is also about 10 ounces lighter than your D850. Basically the same sensor and IQ. Its AF is not as good for BIF and action as the Z8/Z9 and -- I expect -- the Zf. But it is an underrated camera for wildlife and better than the internet commentary would suggest. I used one, together with my D850, and then with my Z9 for wildlife, until I got a Z8 to go with my Z9. Another issue is that the Z7II has been out for a while and may be soon replaced by a new model (possibly a Z7III or maybe even a Z6III, depending on specs).
On the lens front, the Z 100-400 is the closest in focal length range to your 80-400 (which I used with my DSLRs). It is a bit lighter than your 80-400 -- I think about 6 ounces (assuming your 80-400 is the AF-S version). I find its focal length range and close focus ability quite useful. Optically, I think it is a bit better than the 80-400.
A lighter weight option is the Z 400 mm f4.5. This is about 15 ounces lighter than your 80-400. It's very nice to shoot with. But not a zoom if you need shorter focal lengths. If you need longer focal lengths (especially for birds), I find the Z 400 mm f4.5 takes the Z 1.4x TC very well and is also good with the Z 2x TC (of course, you add a bit of weight and lose a stop or two stops of aperture respectively). I have used this lens bare and with the two Z TCs. Like it a lot. Nice and light on a Z8.
I have the Z 180-600 mm lens. It's nice and versitile, but it is about 18 ounces heavier than your 80-400 mm, so I don't think it would help on the weight front.
Nikon does make a Z 600 mm f6.3 PF lens. It weighs about 4 ounces less than your 80-400 mm. Nice reach and also plays well with the 2 Z TCs. But not a zoom.
I think that some of the Z lenses have been made with the weight closer to the camera mount, which helps on balance. You'd likely want to hold the various alternaives and see what they feel like to you.
You could also try a lighter F mount lens. You could use the F mount 300 mm PF with an FTZ adapter if you go with a mirrorless body. Nice lens. I used it on my D850 and D500. WIth the adapter, it's about 25 ounces lighter than your 80-400. But only 300 mm (it does take the F mount 1.4x TCIII quite well to get to 420 mm f5.6) and not a zoom. The F mount 500 mm PF weighs about 5 ounces less than your 80-400, but would also need the 5 ounce FTZ adapter to use on mirrorless -- great lens. I sold my F mount PF lenses when I decided to get the Z 400 mm f4.5 and Z 600 mm PF lenses.
Good luck with your choice.