"and that since all popes and most sacraments are invalid."
They say that the Novus Ordo Mass is invalid, as is the sacrament of Holy Orders and Confirmation. So is confession, because there is no validly ordained priest. I don't think they say the same for extreme unction. Baptism and marriage are valid.
Their position on the validity of sacraments is deduced from the change in the rites established by Paul VI, not because of the Council.
Instead, they say that popes after the Council are not popes, because a pope cannot officially promote an erroneous doctrine.

I think the idea that Papal Infallibility, a dogma declared at Vatican I, applies broadly to papal pronouncements. The dogma is, however, very narrow. Such infallible pronouncements have to be worded in a specific way. With the exception of JP II’s declaration on female clergy, no such pronouncements have been made since 1900.

On the other hand, the Church of Vatican II is in schism from the prior 1900 years. The documents of Vatican II can be interpreted in an orthodox way but because they can also be interpreted in schismatic ways, those documents are not orthodox.

I need to learn more about the sedevacanists. They seem to rely on part on a schismatic Pope is an invalid Pope along with some questions about the conclave of John XXIII. By contrast, the SSPX simply doesn’t go there and relies on the requirement that the Pope faithfully protect the deposit of faith passed on. The theology codified dogmatically at the council of Trent and expressed in the Missal of 1962 reflect that deposit of faith. They show respect to the Pope but disregard any pronouncements which fails to protect the Deposit of Faith.

It is fascinating to me that Francis has not used the formulation for pronouncing infallible dogma in any of his pronouncements. Does he realize that the clear contradiction of fixed orthodoxy would bring about a crisis or is it some supernatural protection of the church?