I presume the Z6 operates the same as my Z5 in that the interval timer feature will not work with manual mode. If you used a remote shutter release (ie: use the Snapbridge app) you could just keep snapping photos every 20 seconds and try to make a series of 80-100 images. With that many photos to use in a astro-stacking program you would have a Milky Way with all the details of the nebulae. And you can still layer a long-exposure on top and mask it for showing the foreground subject only.
Thanks, Chris. I check z6ii manual for interval timer. It does not say the feature does not work with manual mode, only this: "Exposure smoothing will not take effect in mode M if Auto ISO is set to OFF". I don't know if that's the same case as your z5. AW I have a remote shutter release (wired) to do the sequence shots too (but didn't do it). Unfortunately I don't have an astro-stacking program (hard to find such program free and working with my old mac OSX system). I tried the auto-bracketing shots (x5 with 0.5EV increment) and loaded into Nik HDR app. The outcome is a total disaster! Have no idea why at all!!! AW, I'll continue to find better sites for the MW photo and try again.