It sounds like you are more knowledgeable than I am with the internals as I would not take apart my own transmission. The following is purely speculation on my part:
In reference to adding parts, it sounds like your transmission is on its way out more than anything. I wouldn't spend money on any of the items you've mentioned (even though I don't know what those could improve). Trans medic seems to be hit or miss, some people advise it along with a shift kit, fluid and filter, and trans cooler. Others say it is snake oil. In reference to your buildup theory, many people have claimed the buildup in a sketchy transmission is the last thread holding it together. That is why a flush can ruin your transmission - it removes all the particulate and material the trans was depending on to function. Very technical, I know ...
I'd do the shift kit, trans cooler, fluid and filter, trans medic and see what happens.
