When thinking about a mirrorless Nikon and retaining your F mount lenses you have to consider what lenses you have and if you want autofocus to work. The official FTZ adaptor works fine for more modern lenses with their own motor but will not allow autofocus on older AF lenses with the screw feed. There is a third party adaptor but I haven't seen anymore than a casual review yet, where they complained about the noise which to me sounded normal for these lenses. With the same lenses, adaptor and full frame Z mount camera you are probably looking at the same sort of size and weight as the D700.
I recently supplanted my (also beloved) D800E for travel and family pics with a Z50ii. The camera and 16-50mm kit lens are around half the weight and 2/3 size of the D800E with the 50mm lens and then I am not taking an extra 35mm, but I am having to take spare batteries instead as they definitely do not last as long. My bag for travel will still include the (kit) telephoto lens and a prime lens for portraits, lower light and lower depth of field but it mostly I would only actually take 1 lens with me or at a push 2 on days out. I am still very much trying to learn and gel with the camera and mostly it is a me issue around using it as a family shooter and not having a day or time of just really practicing for myself, I haven't taken a killer photo with it yet that makes me think I am doing well with the camera. The EVF is very different to get used to compared with the viewfinder on the DSLR, some things are good as it try to show you what you are going to shoot but you definitely feel that you are looking at graphics on a screen and that the image is not as compelling. The other thing that has surprised me is the cameras own settings particularly ISO which on automatic likes to go quite a lot higher than I am used to but there hasn't been a lot of grain and the images have always been useable. Needing to use higher ISO and lower aperture would be one of the trade offs of the smaller DX sensor but then the camera design and software of a 10+ year newer camera should be offsetting this.