If you take the first date, then the formulation of every major Christian doctrine--the divinity of the Son, the divinity of the Holy Spirit, the unity of Christ's Person, the duality of His natures, the need for grace (contra Pelagius), the inadmissibility of the apokatastasis (contra Origen), the perpetual virginity of the Theotokos, the duality of Christ's wills, the duality of Christ's energies, the defense of the reality of the Incarnation by permitting the depiction of Christ in the Holy Icons--are all 'after the canon was complete'. If you take the Orthodox understanding of when the Church fixed the canon, all of those doctines except the first three listed took place 'after' the canon was closed even in the later sense.
Besides the Proto-Evangelium of James, which, while not included in the canon of Scripture was never condemned as fraudulent or heretical by the Church as were the so-called 'Gospels' of the gnostics, there are patristic testimonies to Mary's perpetual virginity before its solemn declaration by the Fifth Ecumenical Council, and before the 'closing' of the canon of Scripture in the other sense--the conciliar decision as to which books constitute Scripture. St. Irenaeus of Lyons in the early third century, and St. Athanasius of Alexandria (whose letters also include the first extant list of the books of the New Testament without omissions or additions), both attest to Mary's perpetual virginity before the action of the Church which closed the canon.
There is a principle of Scriptural exegesis, which I though, was still well-established among protestants, that one ought not intepret one part of Scripture so as to be repugnant to another. Insisting that 'firstborn' implies subsequent births is repugnant to the sense of the Old Covenant law concerning firstborn children: the redeeming of the firstborn, who otherwise belonged to God, was not predicated on subseqent births.
Only if you add to the canon of Scripture the un-Scriptural notion that the Scriptures are a complete account of all that is true concerning Our Lord (do read the end of the Gospel of John), and that anything which cannot be proved by discursive human reason from the surface meaning of their text is false, can you reason validly to your conclusion. Of course, if you take that position, you have to explain how the Church, whose truthfulness and reliability in establishing doctrine you are disputing, managed to correctly collect your complete account. Do you claim the Holy Spirit left the Church between the Fourth and Fifth Ecumenical Councils? (or the Council of Carthage and the Fifth Ecumenical Council, if you like the Latin account of the fixing of the canon better than the Orthodox)? On what evidence?
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