Have you considered surgery? As we age, recovery times and projected outcomes become less appealing. The best day to fix an ortho injury is yesterday.
My concern is the recoil causing exacerbation of the issues, but moreover, inability to shoulder the weapon points to many other complications in your daily life as well, and fixing this is honestly more about enjoying life than shouldering your benelli.
*complex medical history may make the above moot.
If you send me your address, via PM, I will mail you some spare slings. Assuming you live in the continental US. Keep the one that works best, and mail the others back to me.