Also I write this to everybody suffering from this kind of pain. It's the raging epidemic of our time.
This all sounds too familiar for fascia as the source of pain. Fascia pain is often described as "nasty". MRT doesn't show problems is also indicative of fascia.
I may say that I saw your profile pic on LI and immediately noticed your hanging shoulders. Slight head-forward-posture also doesn't surprise if I may say so and if I'm not mistaken.
My advice would be, leave the stretching behind. Theraband is an insult for every young healthy individual. I sorted out my problems by training upper limbs with weights (start with 1.5kg dumbbells), resistance band and also the occasional fascia roll aka foam roller.
Let me suggest some reading to get into the matter.
Beginner level: Fascial Fitness, Schleip Expert level: Anatomy Trains, Myers
Also: Built From Broken, Hogan
With kind permission, from that book:
Because posture is what you do most of the time, it is often at the root of joint pain, muscle imbalances, and injuries. On top of that, it tends to worsen with age. If you don’t address posture, you’ll be continually slapping Band-Aids on the problems caused by it. It’s a game you simply can’t win. But you can’t just fix posture by consciously holding yourself differently. It takes structural changes to musculature [...]. It requires a combination of training your body to maintain better posture and consciously working to hold yourself in better positions. [...]. While exercising, you train muscles and movements that improve dynamic posture, and you see a little progress from week to week. It’s a painstakingly slow process, but it works. And it’s worth it.
Same source: It isn’t the mountain ahead that wears you out - it’s the grain of sand in your shoe.
Good news is that you will get better soon if you start exercising regularly. Bit of a downer is, that it will take about the same amount of time to revert to "pristine" posture as it "took" you to imprint your current posture into your fascia.
Pain from the 50th repetition of a weight lift has far better quality to it than chronic pain.