Alteration to give chest support now makes back look bad

With the picture you provided of your back after alterations, your back still looks fine. Any issue you have may be small enough not to require more alterations. Another option could be to use tape to help make slack on your back.

I would ask for the buttons to be removed & add in a small corset or eyelets with a ribbon to tie because you can get a spot on fit over zippers, buttons or other closures. The first dress I ordered came & was way too big so we removed the zipper & added a corset back. You could definitely take those buttons out & add eyelets & ribbon. Another option could be to remove the buttons entirely & add a piece of textured elastic the same color as your dress so it laid smoothly against your skin but also allowed for structure, flexibility, & stability to hold the straps together so they stay up.