Yes the truly cascade-based solution is the one that I initially assumed you’d want, back toward the beginning of this thread…
But now you will need to change the body tab element back to a basic transclusion of the text field (not itself appeal to the cascade condition — otherwise, you get some kind of regress of a body inside a body inside a body).
So, you want instead to have the template (the whole set of tabs) display only as the body area (some or all) non-system tiddlers. All good; that avoids the risk of bricking your import dialog and other essential parts of the wiki.
However, you then also need to REMOVE that stylesheet that makes the body field disappear, as quoted in your first post:
title: stylesheet
tags: $:/tags/Stylesheet
body: """
.tc-tiddler-body {display: none;}
"""