I had the opposite experience with MacOS. I hold every OS I've used since the early aughts up against MacOS 9 and they are all lacking in terms of keyboard navigation. Maybe it was because I had the previous 10 years to practice, but I felt I could do almost anything in pre-OSX MacOS with the keyboard, only relying on the mouse for application-specific stuff like photo editing. Navigating, filtering, opening files and folders were all incredibly easy.
In KDE it's pretty much a joke every time I have to save-as. Can't even get through that filesystem menu without a mouse unless it supports <ctrl>-l. Dolphin is slightly better, especially if you enable the console pane to make it easier to switch to the command line, but it's still way behind Apple's finder from 1999.
IIRC for the most part option-f would bring me to the file menu. I could down-arrow to scroll through it or hit the right arrow to move to the next menu. In that way I could navigate pretty well.
In KDE it's pretty much a joke every time I have to save-as. Can't even get through that filesystem menu without a mouse unless it supports <ctrl>-l. Dolphin is slightly better, especially if you enable the console pane to make it easier to switch to the command line, but it's still way behind Apple's finder from 1999.