@andyfitz I used to do more backbending before I got old, though most of it was in the lumbar area rather than the thoracic. However my experience would be that in BB's like in the photo of 16 May, there is such a strong force along the length of the spine (from your body weight) that it is difficult to encourage much bending. As well as BBs, some of my experience comes from forearm balances in a backbend (scorpion?) where I found that because the back was supporting the body weight, I couldn't much enhance the bend at the thoracic level. In contrast, I found exercises such as you posted most recently excellent for opening up the upper back - though it is also limited by tightness in the arm muscles. I have a small tight-bend baby whale, that I used to lie back over, grab weights from the floor behind me, and then try to bring my hips to the ground on the other side of the whale, often pressing the feet against a wall nearby to enhance the bend. This produced a very strong bend in the upper back. If my experience is anything to go by, this type of exercise is the way to go - but I never got the upper back flexibility that @Simon shows. This video
shows me 14 years ago (age 62; sorry about the music). Note that the upper back is almost entirely straight, although I am putting as much force on it as I can using the weight of the legs. This type of exercise never did much for improving flexibility, I think because of the body weight taken along the length of the spine. I hope you can understand what I am saying. Another way of putting it, is that a lot of different approaches are needed, because different exercises target different areas in different ways.