To me this text is referencing the brace not the buffer tube. The brace is what needs to be removed and destroyed or made not to work. The brace is the NFA item. Can't buy or sell one without NFA process as of 1/31. This is the grace period for braces currently owned. So remove/destroy or register them.The text:
"removal of the ‘‘stabilizing brace’’ so that it cannot be reattached to the firearm;"
I do think, as you alluded to, that a "standard/original" style round pistol buffer tube would be best if not registering. One with no slots/dimples/holes/screws or anything to attach or adjust a brace or stock.