You asked
Is the nikon z 28 75mm f 2 8 good for portraits?
Here is what the site says about that.
From Nikon Z 28-75mm f/2.8: Nikon Z 28-75mm f/2.8: A zoom lens by Nikon for Full frame cameras, 28-75mm f/2.8. Specs - Maker: Nikon - Focal length: 28-75mm (zoom) - Maximum aperture: f/2.8 - Made for: Full frame sensors - Mounts: Nikon Z - Stabilised: no - Autofocus: yes What it frames like - On full frame (Nikon Z): as marked, 28-75mm. - On APS-C (Nikon Z): like 42-112mm on full frame; f/2.8 gives about the depth of field ...
From Portraits: Portraits: For portraits, the lens matters far more than the camera. A cheap body with a good portrait lens beats an expensive body with a kit zoom every time.
Quoted from this site's pages.