Holy Crow! 29+% NSC?!?!?
It surely has to be the Pergolide helping her.
She is at a point in her life where extra protein can help her a lot.
My coming 24 TWH has equine metabolic syndrome, which causes muscle waste. He gets rice bran and a soy-free vit/min supplement with a pre-probiotic in it.
Some metabolic horses can have soy as their protein source without hurting them, he cannot so I added Standlee brand timothy/alfalfa cubes back into his diet (Tractor Supply carries Standlee. The alfalfa gives him the protein and I have seen a big boost in his energy level - even on last Sunday when the temp was 98 and the "Feels Like" was 114
This fella gets 1 - 1-1/2 lbs twice daily mixed in with his supplements.
My Arab is 25+, is only 13.3H and lost about 60 lbs last winter. It caught me way off guard as he has never dropped that kind of weight in the 18 years he's been with me. He has four molars missing, so I have to be extra careful that his cubes are completely mushed up to resemble wet grass. He gets 3 lbs twice daily.
The Arab also gets 1/8th cup of Soybean meal twice daily. Soybean meal is very high in protein and amino acides but, your cushings horse may not be able to tolerate the soy.
Regarding feed I would not even finish up the remaining bag -- let the rabbits "have at it". If you can get Triple Crown within your driving distance, their Lite or Safe Starch would be the better options for her. Both are complete feeds for these types of horses.
If you can't get Triple Crown, I would switch her to rice bran which is 22% healthy fat and provides a cool energy, plus mush up some tim/alfalfa cubes and mix them together. She would still need some sort vit/min supplement.
If you want to feed her hay cubes, they would have to be well soaked in hot water so you can mush them up to resemble a pile of wet grass. It feels really good between your fingers --- NO it does not
Big Kudos to you for worrying far enough of ahead of winter. You have done fantastic getting a cushings horse to age 32