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It's debatable whether Religious Liberty is heresy in the strict sense or just a grave error, perhaps approximating heresy.

But your attempt to pit theological notes against one another is invalid.

Ecuмenical Councils are not protected merey from teaching heresy, but from teahing grave error of any kind, and some would argue any error, absolutely speaking.  You're almost imagining that the Council is a person, who has to be guilty of heresy in the strict sense to lose membership in the Church (and not of some error with a lesser theological note).  That's a fallacious perspective.

It's also an incredibly myopic view of the situation.

If nothing else happened other than Dignitatis Humanae ... no New Mass, no bogus canonizations, no teaching of religious indifferentism, etc. ... I'm certain there would be no Traditional movement.  Nobody would become a Traditional Catholic over an isolated error in Vatican II.  They would have simply remained as they were, attending the Tridentine Mass at their local parish, sending their kids to the local Catholic school, entering the seminaries and convents ... while perhaps questioning Religious Liberty, etc.

So your attempting to reduce the entire error of the Conciliar Church to the single proposition of Religious Liberty is entirely invalid.

As Bishop Williamson points out, the entire thing is thoroughly imbued with subjectivism.  Even where Traditional truth is being reiterated, it's done so in the context of this being the "fullness" of truth, so it's a RE-presentation of Catholic doctrine in a subjectivist and relativistic framework (rather than as absolutes).

Really, the core heresy is in fact the new ecclesiology (along with the new soteriology) ... and Liberty is merely one offshot of that core error.

But the Conciliar Church really is a new religion, with new teaching (ecclesiology / soteriology), presented in a relativistic / subjectivist framework rather than as absolutes and objective truths, its new public worship (non-Catholic syncretism with Protestants), and its new catalog of saints, many of whom were political selections due to "woke" criteria rather than based on proven heroic virtue ... not the least of which were the degenerate V2 papal claimants, the greatest wreckers of the Church in history.  Never has a series of popes wrecked the Church so badly and yet not since the time of the early martyr popes have we had such an unbroken series of "saint" popes in the history of the Church.  We really should have set the world on fire with Catholic faith with such saintly popes at the helm.  No, they were canonized merely as attempts to canonize Vatican II itself, which is a new Church, representing a rupture from the old ... where many of its adherents believe that their Church was founded at Vatican II and ignore anything prior to it.