The internal structure is critical to getting the fit right for a dress. The more formal and complex the outside the same will be true for the inside.
There is a vintage technique I commonly use for formal and wedding dresses and that is adding a waist tape.
the waist tape is a strip of rigid ribbon placed inside the gown at the narrowest point. I love using it for a few reasons. If the dress is tight it will be closed before the zip making it easier to pull the zip (or buttons) up. keeps the dress sitting well and not riding up. It can be raised to around the ribs giving more support for a strapless dress or larger bust.
I have added some images showing the inside of vintage gowns where a waist tape is visible. Before the 1900’s it was used to keep the bodice in place over a corset, gowns were usually made of a top and skirt and the gap between should never be visible. Later it was used to keep the waist of the dress close to the tiny wasp waist in fashion in the 40’s and 50’s.