The framework looks on the road to officially supporting NixOS, on top of just being a genuinely good idea: Exciting Partnership Announcement: Framework Community & NixOS Communities Join Forces! - #16 by RaitoBezarius
You’ve mentioned it, but thinkpads have an excellent history with Linux support, and in my circles look to be the brand to go for software developers, especially those running Linux.
Outside of that, check this page, it effectively lists vendors and devices that meaningfully support Linux - if they’re not on the list, firmware updates are too painful to do since you’ll need to boot Windows, so you’re not getting support from your vendor: LVFS: Device List