From my limited experience with the 1DX III the one reason I would upgrade from my MkII is the AF button improvement. That little infra red thing is an absolute killer addition for fast AF point movement.
I’m most definitely not an expert when. It comes to action photography, nearly all of my experience has been manually focused landscape work. I’ve done some of course, but never consistently and therefore not at the level of quality one can come to expect with experience and practice.
That began to change a little with the 5d4 body (5D and 5d2 previously). I do have the ole eos 3 which had unique tech for its day. In any event it was when I picked up the m62 that I really started choosing different subjects and shooting in nearly a daily basis. I know for a fact that being able to co tell focus point location by dragging my thumb across the back screen was a large part of that. I have the r5 set up the same way.
I realize the 1d series is and was on a different level from an AF standpoint, but even with the inherent drawbacks to and EVF vs OVF it seems impossible to deny what mirrorless tech has done for autofocusing.
And speaking of EVF use, I actually shot a long exposure image with 16 stops of nd filter hanging on front. What’s amazing was I was able to compose AFTER placing the filters because the evf allowed me to see! Then I could change comps without taking the set up off! Never happen with OVF, just isn’t possible.
Sure there are things I think an OVF is better for. Following action being one of them. Everything is a compromise however. With one you can follow action easy but achieving focus where you want it is at least more difficult. With the other you can focus just about anywhere but following is more difficult. Pick you poison.
***Apologize for my grammar and spelling, phones are not the best places to compose responses.