Really nice Flickr page, John. Beautiful landscape photos!I skipped the MkIII, but I sold a Mkii in late '16 or early '17 that had maybe 20k clicks on it. I shoot almost entirely landscape so it just doesn't get hundreds of clicks when I get out. It is very common for me to head out for a morning and come back with <40 shots. I hear you on the front/back focusing thing. I'm fortunate enough to have not dealt with that kind of thing a whole lot using manual focus lenses. I do have the 70-200 f4L and the 100-400 v1. I remember calibrating AFMA on the mk II but I"m not sure I've ever had the lens on the Mk IV. I use the 70-200 for telephoto range landscapes.
I think the R is a great body and would work great for what I do with them. I'm not sure it gives me a lot/anything over the 5d4 for the way I use the camera however. If I were still sitting with the MkII or the MkIII I would probably make the change now.