Not disagreeing with anything you say but related to the “can make minor improvements “ comment. I can’t recall the players name but a Sydney mid of about 20 years ago worked with a running coach to make changes to his gait and the improvement to his speed elevated him from foot soldier to one of the comps top mids. Be a better point if I could remember his name .Agree speed is something you have or you don't. Can make minor improvements with work, can't make a plough horse into a sprinter.
Can certainly improve on endurance though which will allow getting from contest to contest to improve. And be more effective in defensive running.