If it’s hard to breathe deeply from the diaphragm, don’t just look at the posterior chain. You’d benefit from also smashing your gut on a ball for at least 10 minutes two or three times a week. For me, the Supernova is just the right size but you can use a kids’ ball from Target if you have too. Lay face down and try to create shear by moving across the ball. Contract and relax. Get right up in there along the base of the rib cage, and all the way down to the top of your legs (tip, don’t do this after a big meal or drinking a lot of fluids!). Also hit the obliques with your mobility tool of choice, and then go all the way up the lat seam to the shoulder. Your trunk is the carriage for the breathing mechanism and tightness anywhere impacts your ability to breathe correctly, let alone the havoc soft tissue restriction can wreak on mechanics. Kelly has posted the gut smash over and over in the Daily RX for a reason. Like the couch/super couch it feels uncomfortable, but it may well help fix your issue.