There are no replacement seats that will provide a simple bolt-in solution. Orthopedic seats were an option on 1980s Mercedes, but they are rare as hen's teeth.
A friend's dad did put Audi 5000 seats into his older, w116 SD but it was quite a project.
I can't imagine how big you must be to have a shoulder issue in a w126. God help you if you get into a w124. I am 6ft 4ins and more than 400 lbs and I have ample room in an SD of any vintage.
The seats in the car you tried obviously need rebuilding. You shouldn't sink into any Mercedes seat. But they will never be like a fine piece of furniture. If you want that, you should try a Volvo XC70, 2003-2007 vintage. They are troublesome cars but they are very large inside and have superb seats.
Or, of course (shudder) an SUV.