Well, as they say, "if it ain't broke, then don't "fix" it." The current feeding system has been working for two/three years.
The grain she is has, according to Purina, "a unique combination of advanced supplements", so I was wondering if I do cut out the grain, if it'd make much of a difference.
She's getting barely anything from the grain, if you're only feeding 4 ounces a day. A half cup is 4 oz, and for a complete feed that is not really doing anything at all except giving the horse a treat. If I put that into the FeedXL calculator, the grain you are giving is providing somewhere around 1% to 7% of the amount your horse would need of most vitamins and minerals per day. An example would be vitamin E. The amount of grain you are feeding provides 40 IU of E, and horses need at least 1,000 IU daily.
It is good you are analyzing the hay, because her hind end muscling looks a bit weak, so I'd wonder if it had enough quality protein in it. Over the winter when horses tend to need more hay, they also can end up vitamin e deficient since it does not store well in hay and they only can get enough when they are on green pasture. Shortages of vitamin e and magnesium can affect the muscle development.
I'd say the amount of grain you are feeding would be the equivalent to the amount you might feed of a concentrated vitamin supplement. To me that would be a better idea, since the horse would actually get value out of it to make it worth buying. Feeding 4 oz of a multivitamin such as horse guard would give your horse 1,000 IU of vitamin e as well as good amounts of other vitamins and minerals such as potassium, vitamin A, magnesium, etc. That is often a good way to get horses complete nutrition if they are easy keepers.