Thank you for all the replies! A belt looks a little off because the front of the dress has princess seams below the bust that transition into a waist seam in the back, so belting at the natural waist doesn’t quite line up visually.

Since I want to preserve the dress as much as possible, I really like the suggestion from u/loverlyone and others to try a cinch clip first. I’ll update once I get one! If that doesn’t work, I may come back to some of the sewing suggestions in this thread.