Society has taken advantage of this in historical times by flaying, beheading, or hanging corpses. The idea is that despite the fact that these things have zero impact on a living individual who can feel them

That line of reasoning is incorrect, I believe. I can imagine seeing someone I love treated that way: that would be distinctly worse than knowing they died painlessly and were treated with respect.

The reason is that among my preferences is the wish that my family, friends etc. be treated at least respectfully by others. If this kind of fate is visited upon them, my preferences are being thwarted, which causes me to suffer.

I also wish to continue caring for my family, so from my perspective my own death isn't "something you can't care about because you are dead", it's a big disruption of my plans. That is unchanged by the fact that after my death I will be unable to feel anything about that disruption.

So, concern for what happens to my body in and after death is actually quite legitimate, and in fact is an extension of the legitimate wish to live.

We tend to place great store in people's last words and final wishes.

Well, unless those final wishes are to be stored in LN2 in Detroit. That seems to contradict your thesis, appealing as it looks at first glance.