He did convene the 1stt Council of Nicaea. One of the products of that council was the Nicene Creed. I think that was significant,
While Constantine ordered the council of Nicaea, he didn’t run the show. He told the self-described Christian authorities, bishops from across the empire, to settle certain differences they had once and for all. At the time, there were disputes in the church over issues as profound and obscure as the relationship between the human and the divine in Christ and as quotidian as figuring out when Easter would be. As a ruler over these Christians, Constantine had a valid interest in the end to such disputes but no interest in any given end. The arguments and conclusions reached were those of the Christian church at the time. Constantine is the one who required a settlement of these issues, but did not dictate what that settlement would be. The council of Nicaea was Christians producing answers for Christians. One might dispute those answers, but you can’t really call them Constantine’s answers.
of the 300 odd bishops who came - all agreed the Nicene creed, except one Arius an heretic unitarian.
Arius’s position that the pre-incarnate Christ was a created being, albeit the first of and superior to all the angels, and in fact the creator of everything else except Himself) was unanimously rebuked as novel and heretical. Where the conflict came was in then trying to explain exactly what the Son is in a way that precluded both Modalism and Tritheism.
Constantine had nothing to do with the Canon.
As for Nicaea though, Constantine did call the council because the Arian-Athanasian conflict — as in wide spread “disturbances of the peace”, rabid demonstrations, and occasional riots — was really starting to get out of hand in Egypt, Cyrene, and Cyprus, and was starting to spill over to the rest of the eastern half of the Empire. Quite understandably, Constantine wanted to get it to stop. He called the Council and even paid for the bishops to travel there.
Constantine also presided over the Council— and as very impartial if fascinated judge/chairman, who’s only agenda was peace and unity — as in, “Can’t you guys just get along?”
In the end, Constantine actually suggested the compromise term “homoousion” for the Creed … though he and most everyone else backed away from it within the year when it proved too controversial and too problematic. He and his successor spent the rest of their reigns telling the pro-homoousion hardliners to shut up, keep serving Communion to those who would not confess it, or get out! (As in, “You are now exiled to a one-horse dorp in the Crimea until you are willing to adhere to our policy of toleration and inclusion.”) Increasingly, Constantine sided with and patronized Semi-Arians (and the homoiousion — Yes, the Church was literally tearing itself apart over an iota) and was ultimately baptized by one.
As often happens, the compromise term that was meant to bring clarity and harmony brought the opposite. The term homoousion had a colloquial sense, “same stuff” or “same property”, and by extension “same jurisdiction/right”. But in Aristotelian metaphysics ousion was a special term that basically meant “whatever it is that is intrinsic to something that is what makes it what is and not something else” — the equivalent of the Latin “sine qua non”, “that thing without which it is not”. And attaching “homo” (same) to it creates the same problem in Greek that words like same, identical, and equal present in English: if someone says that A is identical to B, does that mean that A and B are just 2 different names for one entity, or does it mean that A and B are 2 entities that indistinguishable from one another? Homoousion as intended in the Creed basically said, no more and no less, “Whatever it is that is intrinsic to the Father that makes Him “God” is also intrinsic to the Son.” That’s what it was supposed to mean. But masses then were no more familiar with the abstract categories of Aristotelian metaphysics than are people today.
It was not clear to people what homoousion (same substance) actually meant and entailed — some took the view it was Modalistic and therefore heretical; others understood it to be saying that the Father and the Son are made out of “stuff” and it’s “the same stuff”; and no matter how much its defenders tried to get them to understand that was not how they were using it or what was meant by it, multitudes were simply not having it and multitudes more thought it was wrong to excommunicate people who refused to confess something so subject to misunderstanding and not stated explicitly in the Bible.