Forget 100% cotton poplin. Look for shirting with elastane (2-5% spandex). Brands like Bravissimo, Pepperberry, or any "curvy fit" label add extra length to the placket and darts in the side seam.
Run a small safety pin inside the shirt through the placket (the folded fabric edge) so it is invisible from the outside. Forget 100% cotton poplin
Throw away the boxy sweatshirts. Take your blazer to a tailor. Buy the damn plunge bra. Forget 100% cotton poplin
Stand against a wall. Place your hands on the wall at shoulder height. Slide them up the wall as far as you can while keeping your lower back against the wall. This opens the chest. Forget 100% cotton poplin