Hi Greg - thought this might interest you if you haven't seen it:
https://www.outdoorphotographer.com/...os-6d-mark-ii/
In my opinion for nightscapes you definitely want both full-frame and modded - the latter especially if you are planning on using tracking for your nightscapes and going deep like this:
https://apod.nasa.gov/apod/ap170530.html
Cheers,
Pete