Shuckhardt was the first that I know of. But he wasn't the only. Both Patrick Henry Omlor and Hutton Gibson likewise were early traditionalists who possessed exceptionally brilliant minds and quickly came to the realization that true popes could not possibly promote heresy to the entire Church. A large number of people who never went along with the Novus Ordo at any point, ended up sede. And now even the Novus Ordo people are reacting to Francis not by asserting a right to reject his doctrine but by going directly to the argument that he cannot be a true pope. Salza is the poster-child for everything that can go wrong when you take the R&R position and then arrogantly assert that any other position is heretical and/or schismatic.
I'm not sure about Omlor, but we can look at what happened to Schuckardt and Gibson and the many sedes who've gone off the rails and lost the faith, just like we can look at many trad non-sede's who've done the same, heck, I'm sure we all know of many who have done this, often times we know from experience with our own family members who have abandon everything holy to pursue a life of sin. So this is statistic is, imo, not worth bringing into this discussion.
It can be argued whether error or heresy leads to the loss of faith and immorality, which is most often (but not always) what I believe is the case, or whether it's the other way around. Either way, if the (still unproven) accusation is true that Salza lost the faith and is now NO, it could be that, he, as you said:
"came to the realization that true popes could not possibly promote heresy to the entire Church" - which personally, after everything on the subject he had to study in detail, I very highly doubt.