Very true, I agree with your points. Just like procuring a file of CSAM, procuring a shrunken head or even something like an elephant's tusk would be imo contributing to a demand for more to be made, as well as perpetuating a culture where those things are desired or even accepted to a degree, which could in turn lead to even more morally unsound methods of producing/acquiring them.
However I would also add that I think even in the hypothetical where accessing or even storing/viewing some CSAM files somehow didn't contribute to any more being produced or shared by anyone, it would still be fundamentally unethical to access it/store it/view it, because while the most clearly abusive component has already happened, continuing to view or use the product of those actions is further violating the child's right to not have themselves commodified or exploited like that, and disrespecting their right to privacy... for the same reason that a peeping tom is violating someone's right by spying on them in their privacy, even if the person doesn't know it happened (except in this case, it's a violation on top of another violation - the child has been exploited, and then people are further violating the child's rights by viewing it).
This aspect of something being fundamentally unethical even if it doesn't contribute to more bad things happening in a measurable/utilitarian sense but in more of a deontological way where the action itself is violating certain moral duties by disrespecting their bodily autonomy, is where I'm coming from by thinking that using/displaying the dismembered body part of a person is unethical regardless of whether doing so contributes to more of that product being created.
I'm recognizing how many of the behaviors we see in adults are reflective of newer, upgraded forms of the same behavjor patterns of babies, despite adults often thinking of themselves as serious and mature. It's funny to think of adults as grown babies sometimes.