With Windows in particular, you absolutely can navigate Windows + Office keyboard only. I do it every day.
Now, third party software, is always going to be all over the place. Stuff that was largely built on Win32 components works fine, but "modern" stylized applications rarely have strong support.
Most things in Linux too - all DEs I have tried have lots of keyboard shortcuts and so do a lot of applications.
The problem is that they are less discoverable and you need to make and effort to get used to using them instead of point and click.
I just wish the shortcuts between the OS and Office were consistent. Most are, but some of the more commonly used ones aren't.
You’re right that lots of Windows apps were designed with Keyboard only workflows in mind. It’s a shame that MacOS has so many points where if you don’t have a mouse you’re out of luck.
There is one major improvement you can do on Mac, at least for menus:
https://varun.ch/posts/macos-keyboard/