I took a brief stab at this earlier by looking at a variety of international bodies that create trails, feedback from other threads, and my own experiences.
It’s a very rough draft and needs some editing and condensing, but I think it does a good job of establishing a truly “excellent” excellent and not getting bogged down in a lot of confusing overlapping lower ends of the scale.
In regards to the earlier comment on exposure, I’d be fine with just dropping exposure from difficulty but not creating a new key and just using hazard=* nodes though there’s some things to sort out there.