I asked System76 if they would sell a Windows system

"Hi DFS,
Thank you for your interest in our products! While System76 only installs, tests, and supports Ubuntu and Pop!_OS, customers are welcome to install other operating systems on our hardware without voiding your warranty. We test against the Linux kernel, and distros with a current kernel will most likely work well.
We can't guarantee that other distros will function at 100% but we do offer a 30 day limited money back guarantee on all purchases, so if there's an issue you can return for a full refund minus the cost of shipping and any damages. You can find more details about our warranty here: https://system76.com/warranty.
I am also including a link to a support article that goes into detail about using Windows on our systems:
https://support.system76.com/articles/windows
Thanks again and please let us know if you need anything else.
Sincerely,
Bill"
That support article is actually very very good.  Seems like a trustworthy company.