So is the key thing the invitation to all bishops? Like you point out, several councils were lightly attended, or had very few representatives from a certain region, etc. But they were still invited, as far as I know.
Is that all bishops in the Church or all bishops in Christendom? I.e. if an ecumenical council were summoned today by the Orthodox, would HH Tawadros or Pope Francis be invited? Or is it only necessary to invite those bishops that are in communion with the Ecumenical Patriarch of Constantinople, for example?
Isn't it a circular to say that affirmation of a Council at a later Council is evidence of universality? That works/makes sense for Nicea 1, Constantinople 1, etc. But Nicea 2 doesn't have later councils that affirmed it, since it is (arguably) the last ecumenical council.