I believe Sedevacantism itself if followed to its logical conclusion, proves Eastern Orthodoxy as the true faith.
Not entirely… but Orthodox aren’t unified on this. Some hold somewhat Sedevacantist position.
For example, an EO saint, Symeon of Thessalonica from a few decades before Council of Florence, wrote the following (as quoted in Meyendorff, et al’s Primacy of Peter):
“One should not contradict the Latins when they say that the Bishop of Rome is the first. This primacy is not harmful to the Church. Let them only prove his faithfulness to the faith of Peter and to that of the successors of Peter. If this is so, let him enjoy all the privileges of pontiff…Let the Bishop of Rome be successor of the orthodoxy of Sylvester and Agatho, of Leo, Liberius, Martin and Gregory, then we also will call him Apostolic and the first among the other bishops;
then we also will obey him , not only as Peter, but
as the Saviour Himself ."
But then there are some that would dispute this. In the end if Pope before Schism had authority “
as the Saviour Himself” then why would there be no authority to include Filioque? Only valid reasoning is that heretical Pope automatically loses office. Does that sound familiar? Because that is exactly what Sedevacantists are saying. Only difference is between deciding which Pope actually still had authority and which did not.
Standard Eastern Orthodox position is that Popes never had such authority. Then anything in-between really.
Personally I’ve looked East quite a bit and have personally come to the conclusion that the Church Christ founded lays somewhere between East and West, both churches being different sides of the same coin.
Well that is position of Catholic Church. Church is Universal- not Western, not Eastern (or at least not exclusively either of those). Only real deal is that we do not recognize those not in visible communion with us to actually be Catholics. Doing so basically supports Protestant notion of Branch Theory- Church is One but it is divided in between X amount of denominations and we are all correct, right and we are all infallible One True Church collectively.
Catholicism does have problems with Branch Theory because it disputes St. Paul’s words of Church being “pillar and foundation of Truth” as well as Nicene definition that Church is “One” as well as our Lord’s words where He has established Church, not multiple Churches. Funnily enough, Sedevacantism does kind of boil down to this because various sects can’t agree on stuff.