I don't know about the Z7 but the Z6 does not feel like a step back in IQ from the d750? Interesting.
OT, the D750 got me settled into the habit of single image HDR with it's incredible shadow retention. I quickly found out the Z6 is more limited here as patterned noise comes with the shadows, so no more single image HDR. I personally can't stand banding. I will be posting a review of the Z6 in the future that goes over this. Shadow banding aside, the Z6 does retain color much better at higher ISOs.
Please define IQ.
I mean IQ across all types of use in this case. The only areas I have seen noticeable difference between D750/Z6 is the Z6 has better color at higher ISO, but introduces noise banding in shadows. Colors can be improved in PP, but noise banding is simply a killer. So, all things considered, I prefer D750 IQ.