You are probably better continuing to use the 6D2, which has 30% more resolution. Just about all of the advantages of a 1D series body over a 6D series are pretty much moot when it comes to photographing the Milky Way with a wide angle prime.
"...my trusty 14mm Rokinon and the IRIX Firefly 15mm both excellent rsults..."
It is more the lens than the body. I have pro buds that use Rebels for astro work. The big difference between a 1 series is build and not IQ. It is a tool that is ready day in and day out no matter what and no matter what is thrown at it. Not on the top of the list for astro work!
"Looking to buy friends 1DX Mll and was wondering ..."
... was wondering whether you should buy it? Of course you should. Once you go 1 series you'll never go back. There is 1 series and then there is everything else. They are in a class all their own.
09/26/2023: New firmware updates are available.
08/18/2023: Canon EOS R5 C training series is released.
07/31/2023: New firmware updates are available.
05/31/2023: New firmware updates are available.
05/18/2023: New firmware updates are available.
03/30/2023: New firmware updates are available.