Short of getting a dedicated bolt action or single shot slug gun with a stiffer barrel mount, you've done about all you can to the gun itself. Good advice here on scopoe, mounts, and trying multiple slug brands.
I agree with previous post, while I have an 11-87 and rifled barrel I switched over 100% of my hunting days to the muzzleloader years ago. Better accuracy, less recoil, much much cheaper. Longer range too.