>there seems to be no mature GUI keyboard-first UX concept.

I don't know about 'first' because I don't see a reason to privilege keyboard workflows over mouse workflows given how ubiquitous the latter is but Windows has had, for literally decades a 35 page Keyboard UI spec. They've put a fair amount of effort in applications being navigable in a consistent way.

https://learn.microsoft.com/en-us/previous-versions/windows/...