A few contributions regarding the Z6ii in combination with Reflex Nikkor 500C or older PC lenses.
I got the 500/8C in mint condition from a retiring colleague who also moved to a new home at the same time. He used that lens in the eighties in combination with a FE2.
On Df, it was barely usable ; even with a monopod, setting the focus with that perpetuum mobile in the OVF was painful. Using reflex lenses on the stabilized OM-D was slightly better, but with the 1000mm "perceived" FL and the limited high ISO capabilities of the m43 system, the situation was hopeless.
On Z6ii, the stabilizer works decently. I used the 500/8 C handheld without any armrest, and on fixed subjects, focussing is best done using magnification (focus peaking is not precise enough). With magnification on, the image in the OVF is still shaky, but precisely setting the focus remains possible. Also, the EVF has suitable luminosity.
Bottom line, usability of the reflex Nikkor is increased, but I still prefer the 300 PF, despite the lower magnification, from every point of view (contrast, sharpness, ease of focussing, bokeh of course, vignetting...). The 500/8 C is usable though. Here are a few examples.
