I think you're trying much too hard, tbh. Just be yourself. Never fails!
I hsntly don't think there are integral garments that add an aura of 'demure sophistication'. It's all down to the person wearing the particular garment, rly. I've seen incredibly recondite women in absolutely kooky wares look nothing short of elegant. Same token, I've seen women in unremarkable pieces look absolutely incredible, too. You have to figure out what suits you and you also must establish a bit of comfort in your own skin. As you already pointed out, perfect posture is a necessity. I also think a cheerful disposition, genuine kindness, a dash of mystique, 'joie de vivre' and good manners go a looooong way in the sophistication department, aswell.