Hi Laura! First let me say she's a beauty! You didn't mention how old she was so I'm going to assume she's 4 or 5 years old (?).
Will she be on any type of pasture at your place? Do you have an arena or round pen to ride in, or trails?
That all said, here's my 2 cents worth. Take her off the sweet feed. She obviously doesn't need it and is not worked hard enough to justify the extra sugar. If she's only on hay 2x's a day and a scoop of sweet feed now and looks like that, she's what we in Maine call an "easy keeper" . If she's going to go into a pasture, limit her time until accustomed to it. If not just hay her and considering the amount of work she's going to do for you, may not need any extra feed.
If you want to give her something, well, a complete feed would probably fit nicely, though I've always been if a horse keeps their condition on so little, keep it simple kind of gal.
As far as getting some weight off her. By eliminating the sweet feed that should help some. Next just walk and trot her. It's amazing how some regular nice long walks can take weight off and condition a horse without putting too much stress on their joints. I like to mix it up with walk/trot combos. No hills, not a big deal. Many of the horses at the barn mine were at were ridden only in a large arena. They were fit and muscled. Just ride. Like someone said on this forum, the best thing for a horse are lots of wet saddle pads!
Hope I was of some help. Have fun!