As best as I can tell a Z7 upgrade would net a 3/4 pound weight loss and a pretty big resolution increase. I'm hesitant to dive into a brand new system in the first generation.

If I were starting from the beginning I'd look at an A7RIII and a few key lenses, but I'm not launching a major expedition and learning a new system is quite painful. I'm in decent shape, having large, small, long, wide and fast options in a single system. While not 135 format it has a very effective high-res mode that, however, requires a tripod or some support.

Good luck!