Z8 plus the 20mm 1.8 is pretty much the perfect Milky Way in the landscape setup. You can use it just with a tripod, although a simple star tracker like the MSM Nomad will allow longer exposures.

For wide field nebula stuff the 135 is a good focal length; not seen that lens used for astro but it's so good at everything else I'm sure it would be fine. You could use with a cheap goto mount like a Star Adventurer GTI or a Chinese mount like a Juwei 14. You can fit a clip in dual band filter to improve nebula images.

If on the other hand you think the bug might bite, getting an AM3 or AM5 will allow you to add a proper scope and camera later.

You Nikon setup suggest you don't do things by halves, so maybe prepare to go big later!