Try something different with your back and shoulders
I am not a doctor but I had issues with my hands and shoulders that felt like torn rotators and at times a broken wrist. I could not ride a mile and would have to stop to let the pain subside so I could turn around and go home. I had an MRI of my neck and found a narrow section that was pinching nerves and causing the problem, the cause of that was sleeping face down and lying in bed reading face down. I changed that behavior and after one week of the proper physical therapy to treat the issue I was back on the bike for 30-40 mile rides and after a full 8 weeks of the correct treatment I had no problem with 300-600 mile rides and on a variety of bikes from the stable.
Here's my hijack
1955 R 50, '62 R 50S, '65 R 69S(grey), '68 R 60US(original red project), '69 R 60US (Dover white), '71 R 75/5, '73 R75/5, two /5 bikes in progress , '75 R 90/6 X 2, '75 R 90S (restoration in progress), One /6 bike in progress '79 R 100S, '97 850R rework, '01 Kawasaki W 650 X 2, 04 GSA, 06 HP-2 Enduro, Thruxton, Bonnie SE, Scrambler and I have a deposit on the R Nine T. I've bought and sold a few in between and miss all.
I hope you try something different to see if you can sort out your back issue and get back on two wheels.