I have no such problem. By the completion of the Canon, I mean when John wrote the last letter of Revelation around 90 AD. EVERY MAJOR DOCTRINE all Christian essentials were contained in Scripture. The divinity of the Son, the divinity of the Holy Spirit, the Unity of Christ's person were all complete in Scripture whether or not man got it at that point or not (which he did- only schismatics like Arius denied some of these things).
The problem with some of the newer doctrines is not that there is suddenly a fuller understanding of Christ but that the doctrines coming out actually contradict and twist Scripture. Mary was not a perpetual virgin. Mary had other children. Nothing was sinful about either aspect. But she was also a sinner. She called God her Savior. One has no need of a Savior if one is sinless. She is NOT a co-redeemer. By that same logic, So was John, so was Peter, so was Pilate for all "suffered" in some aspect in the death of the Savior.
The early church, even in Scripture, had its problems. It was not inerrant otherwise Paul wouldn't have been writing all of those letters to the various churches. The Thessalonians thought they were in the Tribulation. The Galatians thought they had to be circumcized to be saved. The Corinthians were FULL of problems.
Of course, the Holy Spirit breathed and directed the writing and formulation of the Canon of Scripture. But there were various doctrines along the way, such as Pelagianism, Arianism, Gnosticism, that were not Scriptural or inspired by the Holy Spirit. When things contradict Scripture, they were tossed. The same standard should apply today.
God preserves his Word, but the church of Christ is not perfect. If something contradicts Scripture or can not be fully supported by Scripture, toss it.
SOLA SCRIPTURA SOLA FIDE SOLA GRATIA SOLOS CHRISTOS
But you still have to rely on the Church's judgement as to which books are Scripture. What ever criterion, other than the Church's judgement, you purport to bring forward ('undoubted apostolic authorship' is the usual one you sola scriptura types usually propose), you have no criterion for verifying that the canon of the New Testament and no other books satisfy it other than relying on the ancient Church's judgment and the long tradition of the Church.
If you trust the Church to have judged rightly which books are Scripture, why do you distrust the Church's judgement on other matters?
Again, what evidence can you set forth that the Holy Spirit led the Church correctly to fix the canon of Scripture at the Council of Carthage and the Fourth and Sixth Ecumenical Councils that ratified that local decision, the former implicitly in the phrase 'ancient canons', the later explicitly, but that the Fathers went astray at the Fifth Ecumenical Council?