Yes, the exercise ball helps. I use it in one of the thoracic exercises I do. The key is get proper training on this though, because it’s the upper back you want to train not the lower back. I know in wrestling when I was young we would bridge up to avoid getting pinned, but that was often pushing a bad habit of arching the lower back.
You see, the lower back isn’t designed to be very flexible whereas the upper back thoracic is. However, years and years of sitting at a desk most people’s freeze up and really have to be worked at to get flexibility and mobility back in it.
I’m sure youtube has some free proper training guidance on it, but I am doing the training program of the link offered on it in my orginal post. Here’s a free youtube link that looks good, except the person doing the training demo is a gumby so that is demoralizing, lol. Back Bridge Training I can tell you the training for stretching my thoracic is 40 minutes long. The reason it’s so long, because he warms your upper back up, and works mobility issues out as well before really getting into it. Once a week, but that quite a intense experience.