For the sedevacantists here, what materials would you suggest I read/ watch regarding sedevacantism and what was it that brought you to the sedevacante conclusion?
Heretics are not Catholics. Catholics are baptized, confess the faith undefiled, and are subject to the (to any) reigning pope. Heretics do not confess the faith undefiled and hence are not Catholics.
The robber council of the 1960ies was a most solemn declaration of heresy of virtually all bishops of the time including the pope. E.g., to hold that there is a natural right to religious liberty, is heresy. All Fathers of that robber council are heretics having declared their heresies in a most manifest way. Their offspring is of the same kind.
Heretics are defectors, which — following Canon Law, Can. 188 §4 — lose their offices ipso facto
(sentenced by the fact itself), or in other words latae sententiae
(sentence automatically imposed, no further explicit sentence necessary).
But we do not even need Can. 188 §4. Communicatio in sacris
with heretics is a sacrilege. It does not only not please the Lord. Communicatio in sacris
includes participation in their rites as well as their teachings. A pope or a bishop cannot be someone we should shun, whose rites and teachings we must avoid.