If you can do a spray, rather than a straight shoot out of the hose, and spray the coat against the direction of growth, you can get out a lot more sweat and dirt than doing everything with the direction of growth.
700/225mg is plenty for most horses, and nothing short of no sunlight at all is going to prevent some level of fading for the horse with nd1, much less 2 copies, all you can do it reduce it to some degree.
That said, sometimes a horse doesn’t read the numbers and needs a bit more, 300/900 cu/zn, occasionally a bit more. It’s too late for any increase to help this current coat, that needs to start in Jan or early Feb at the latest. So you can see what this Summer brings and if you want to try adding a bit more, that’s fine.