Using Nikon Z6 to shoot star trails

I have no experience with the Z6, but the Reference Manual does have words to try in regard to camera mode M in Interval.

Nikon Z 6 Reference manual, page 214. There is also a manual for a Z 6 II model.

• Exposure smoothing: Selecting On allows the camera to adjust exposure to match previous shot in modes other than M (note that exposure smoothing only takes effect in mode M if auto ISO sensitivity control is on). Large changes in subject brightness during shooting may result in apparent variations in exposure, in which case it may be necessary to shorten the interval between shots.

• Interval priority: Select On to ensure that photos shot in modes P and A are taken at the chosen interval, Off to ensure that photos are correctly exposed. If On is selected, check that the minimum shutter speed chosen for ISO sensitivity settings > Auto ISO sensitivity control in the photo shooting menu is faster than the interval and that Release is chosen for Custom Setting a1 (AF-C priority selection) or Custom Setting a2 (AF-S priority selection) according to the focus mode currently selected

The most obvious workaround is to use an external wired shutter release with a built-in intervalometer. Before cameras started including internal intervalometers, that's the way we had to do it.

Third party units can be had fairly cheaply for around $20 or so. There are also wireless versions starting at around $45.

You set the camera with whatever manual settings you desire, then you set the intervalometer to take a photo at your desired interval.