I mainly shoot Portraits and some Landscape and Family groups. I had the 645Z about 2 years ago and sold it IQ was excellent. My main question is
IQ on the Z vs the Nikon Z7. Color saturation, sharpness Detail. Thank you
I can't speak for the Pentax, but the Z7 is excellent in sharpness and detail. Color is more subjective, but as a long-time Nikon shooter, I like it a lot.
Experientially, I'd expect a world of difference. The Z is about half the size. It is very well-designed for usability, so that's a real plus. I've been using a Sony a7R3 a lot recently, and the Z7 just feels better in my hands. Also, I can find the buttons, and the EVF is really very good.
At 47Mpx, I would think the difference in the resolution part of IQ compared to the Pentax would be unseeable, but there's more to that story. At that resolution, it matters a lot what lens you put on the camera. So both the performance and the character of the lenses should be a major deciding factor. I'm loving some of my AI-S lenses on the Z7--manual focus is very good, and when the sensor out-resolves the lenses, the result is that you get a very good recording of the lens's character, and that can be quite special.
On the other hand, it's easier to design better lenses for a larger sensor, and as I say, I can't speak to the Pentax glass at all.