There is a reason why "Thuc line" is not a stamp of quality.
There is no one "Thuc line" consecration to scrutinize -- that's the problem. There were so many, and some are shaded in mystery, especially towards the end of his life. And it's not just those HE consecrated that are the issue -- how many of them are deceased as of 2021? -- but his "line". The doubt and shadow concerning validity multiplied exponentially as you got further from Thuc himself.
Hence the reason "Thuc line" is a flashing yellow light -- if not a red light -- for most Trads.
There are a number of very dubious and shadowy +Thuc lines for sure, but they don't get any more "quality" than +Guerard des Lauriers' line. That man was the personal confessor to Pope Pius XII and helped draft the dogmatic declaration regarding the Assumption, co-authored the Ottaviani Intervention. Then Bishop McKenna was no slouch either, and from there you get +Sanborn. On the other side, +Carmona and +Zamora were also solid. There's no question about either of these lines ... except Bishop Kelly's nonsense about the mental state of +Thuc because he made some bad decisions on whom to ordain. In point of fact, however, imprudence is no indication that someone cannot validly confect the Sacraments. You'd have to be so far gone that you didn't know what you were doing. Witnesses testify that +Thuc, AFTER these consecrations, could sit at table with priests who spoke different languages and switch effortlessly between languages and keep track of separate conversations. His autobiography (also written later) is very articulate and moving. Basically, so long as he was aware that he was consecrating bishops, those were valid. Nor is there any canonical requirement for there to be a witness who knows the essential form and can attest to the fact that it was correct. If the Bishop was properly trained (and +Thuc certainly had been and had performed other consecrations prior to V2), the canonical presumption is one of validity. Between +Thuc (who had advanced theology degrees) and +des Laurier, it's laughable to suggest that they didn't know enough about the consecration to determine whether it was valid. Fr. Kelly made up requirements for validity out of thin air. Derksen's books exposes him as totally dishonest, taking quotes totally out of context and even using strategic ellipses to make the quote say something different than what it originally did.