I'm with you John ! I also struggle with this to the point of sometimes not actually knowing if the cone bent until the kiln has cooled some, and I can see it. At earthenware temperatures a flash light helps, but not so much with cone 6 . I plan to try dark underglaze on a kiln post behind the cone.
Have you tried terra sigilatta ( spelling ? ) It's beautiful on low fire but unsure if it would work for mid fire. I have also used various wax combinations, even floor wax. Of course burnishing is the right way but takes lots of time