What version of Linux would people recommend for this machine ? 2018 Thinkpad X1 Carbon (6th gen)
Thinkpads are popular in the Arch community (and vice versa), so there's discussion and documentation available if that's your cup of tea.
https://wiki.archlinux.org/index.php/Laptop/Lenovo
It appears the X1C 6th generation doesn't have a page yet, but it's likely extremely similar to the 5th generation one.
That laptop has Intel graphics, which have great Linux support. It will run practically any distro without any extra effort.
But if OpenBSD runs, it’s a good indicator that the hardware will be extremely well supported and work with the absolute minimum fuss, even if running Linux rather than OpenBSD.