Saddlebag, senior feeds *frequently* have added sugar & are commonly grain based, but depends what feed you're talking about. Unless the horse is too old to have working teeth, I prefer to feed (grain free) fibrous feeds rather than pellets that are 'prechewed'.
While oats are considered about the safest grain for horses - easier to digest, lower starch, more fibre - IMO it's best to stick to 'low carb' diets & horses are generally healthier a little on the light side than too heavy. Along with teeth, other problems that could cause a horse to be in poor condition can include stress/management, worms(sometimes regular pastes aren't effective), ulcers, too much starchy/rich feed, too little roughage, nutritional deficiency, IR probs...
OP you talk of lack of topline & doing little work with her, so it's also possible that your horse isn't at all light on, but lacks muscle. Exercising her 'in hand' - not riding - will help that too.
You need to show us the horse for a start - I agree from the little that can be seen in that pic he's certainly not fading away - you need to tell us WHAT you feed, not just % of protein or whatever, & how much you feed(weights, not 'a scoop'). How much & what type hay/pasture she gets too. What supplements, if any, she gets for nutrition, etc.