I think XPath 4 is going to change the semantics of the except
operator, I would need to look up whether your attempted use of except
would work as you intended with the new semantics.

I was wrong, the change in the version 4 specs and with semantics of
"except" is only for XSLT patterns
https://github.com/qt4cg/qtspecs/issues/402, not for XPath expressions.