inguy that I'd prefer a little less shiftiness from the feed company. Why add NSC to the label but then don't give a percentage but just say "reduced?" Reduced from what? 65%?
My guess is that this is not a bad feed, but I don't like the dishonesty. It says grain free, so you would hope that means low NSC and quality ingredients. But there are known cheap fillers that feed companies use, that are grain free but not great nutrition.
Rice bran has a NSC level of 25% on it's own, so if the feed has a lot of rice bran added the NSC won't necessarily be very low, just low "in comparison" to other feeds with grain. So you might say, "reduced."
All of that is fine if you are feeding two or three pounds to add calories. The horse won't be affected much. Same with beet pulp. Sure, the Calcium/Phos ratio is not good in large amounts, but if you're only feeding 3 pounds or less it's not enough to affect the horse's diet. If you feed more than that, you want to add some calcium. I think it's ideal if you need to put weight on a horse to feed alfalfa with beet pulp, because the alfalfa has extra calcium.
It is my opinion that every horse will benefit from a multivitamin. Most diets of just hay or pasture have some deficiencies if you have them analyzed and plug them into a feed calculator such as FeedXL.
Is your horse getting free choice hay? Is the hay high quality? Start there, of course, and also make sure the basics of teeth and parasites (you seem to have already addressed this) are taken care of. I had some friends that wanted to save money so bought cheap, low calorie hay and ended up paying much more for hard feed trying to keep weight on their horses. Many horses, even young and growing ones don't even need extra feed if they have free choice hay. My first thought is to add alfalfa if just grass hay is inadequate, and then some beet pulp, and then some fats, and then finally turn to complete feed if all those don't put the weight on.