Both Vatican Councils radically seem to have radically changed the Roman Catholic religion. Taking Saint Vincent of Lerins’s “Id teneamus, quod ubique, quod semper, quod ab omnibus creditum est; hoc est etenim vere proprieque catholicum” as a measure, perhaps both of you misrepresent “Catholicism”?

Your own reasoning shows that “Sedevacantism” is about much more than rejecting the Pope. It also rejects an entire “ecumenical” council and the “Novus Ordo” rite. The term Sedevacantist could just as well apply to many other sects. If you take offense to “Roman Catholic (Non Una Cum)”, perhaps you could work towards a compromise that is acceptable to both, instead of dishing out stuff the other group finds offensive?

Now, something like “Roman Catholic (Sedevacantist)” seems best to me. Yet, it would be an extremely bad practice if it were up to outsiders to define the name of religious groups. We wouldn’t still call the Assyrian Church of the East the “Nestorian Church”, would we?