This does not work great. What if you have a grid of 30 item, do you press 20 times tab to focus finally the item you want to interact with?

Keyboard design is much more, its also about how and where to place items, how things should move... it's a whole world. In the mouse world we have "discovered" those things and now we "just copy" other designs that we know work fine.

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).