Not good news for the 6D2. Dynamic Range is worse than 6D at lower ISO and at best the same for the higher ISO's. No improvement here.
https://www.dpreview.com/forums/post/59833872
Also it appears to be old tech in the sense it has off chip analogue to digital conversion which latest models no longer have. Perhaps intentionally done to enhance 5D4 sales so if you want to lift shadows with no noise penality like you can do with 5D4 and any Sony or Fuji then you get the more expensive model.
So unless its the swivel screen or the better AF then not much to offer over the 6D for astro. Of course AF is not used for astro so its the swivel screen which is useful.
Greg.