Z6 vs Z7 for IR and astro landscape

[peabrain]

Quick question
a former coworker is telling me to get a Z6 over the Z7 for my IR and astro photography.
My main focus with the modded cam would be.
IR landscape and IR city pictures day and night.
some occasional Ir wildlife shooting with a small tele zoom.
landscape astro photography
and also later some deep sky astro photography.

his main point is "cleaner" image and less noise.

for me i want more resolution to be able to recompose or crop, and when i get a Z7II i will have the same resolution when combining something together
ps i plan to buy used to give a full spectrum mod.
pps. the Z7II will be for wildlife and landscape with occasional night landscape.

[Wintersun]

I bought a used D610 for modification by the people at Life Pixel (https://www.lifepixel.com/Lightpixels). The D610 has live view which is very helpful for IR photography.

Mirrorless may have advantages for astrophotography with the electronic viewfinder but I dislike the EVF for use outdoors in sunlight where the brighness shifts depending on how close my eye is to the EVF. 

In general low resolution cameras have larger photosites and are more efficient at capturing photons of light. Lots of other variable but I would expect a Z6 to be better than a Z7 for astro use. I recently acquired a D5 and am curious as to how it will compare against the D850 for astophotography. One advantage of the D850 is that in APS-C or DX mode it still provides a 19MP file, much as does the Z7 camera.