The most common accuracy problem for handgun shooters that have some background in shooting other guns (particularly rifles) is lack of follow-through. Believe me, I understand your concerns with sight picture and aging eyes, but have you tried dry-firing your 686 and CAREFULLY observing for movement of your sights during/after you release?

S&W revolvers generally are among the handguns that are easier to follow-through with, since they have a fairly fast lock-time and usually a pretty good release or break, when fired single-action. If you can maintain a fair group when firing from a rested position, I would investigate the follow-through issue and see what you find. Tiny movements of your gun during/after release are a dead giveaway that there is a follow-through issue that needs attention.

It is usually the case that in aimed fire that is not taken in any particular rush, sight picture niceties are more a matter of preference than a big disparity in group size.