I don't think this really addresses GP's comment as far as I understood it.

Your resource mostly describes how to make keyboard-based workflows accessible. But just because the full functionality of an app is keyboard accessible, doesn't mean that it works as well as the pointer-based approach - especially if you have an app with multiple menus, sidebars, headers, footers etc. Maybe you could tab through everything, but a pointer will be faster. This is where a "keyboard-first UX concept" is missing (from GP).

I think an interesting idea to solve this would be a "focus navigation mode". Enter this mode with 1 shortcut and then navigate between items with a few keys; like a combination of mnemonics and screen reader navigation e.g. jump between headers with "h" etc.

That's not what I mean. The site you linked is about how to make a mouse-first website more keyboard friendly, but what I would like to exist is a mature keyboard-first guide.

I mean something like this (my wip keyboard-first file manager): https://ibb.co/G3WBW5C1

The hard part is, there seems to exist no UI & UX design language I can follow to create a nice keyboard-first app, so I have to think through everything (which is fun but tiring).