Mirrorless cameras of the same sensor size are only marginally lighter than DSLR. Yes, the bodies are considerably smaller and a bit lighter, but the lenses are essentially the same bulk and weight. (not to mention the cost of all new lenses) To truly save weight you'd have to go forgo full frame altogether, and maybe even DX depending on your personal limitations. M4/3 is out there and very light and compact, though not much to choose from.

If you already own DX lenses you know how much lighter they are than the FX. A D3500 or D7500 with true DX lenses would cut your average weight a great deal over the D850 and FX lenses, probably more than half. While it's no fun going backwards with regard to features, if weight gets to be a real problem, that may be the way to go.