Just from a mod-position, I agree with you. Scripture is the ultimate conclusion. However, if we defer to the whole of Scripture as the standard of who is/isn't a Christian for purposes of flair/Christian-Only posts, then we invite a LOT of other nonsense into the equation, where there are lots of disagreements. Christians can't even agree on which topics are soteriologically significant and not.
The Nicene Creed is not magical. It is not divinely inspired. But it is a clear statement of faith, the same way that any Protestant denomination will have its own statement of faith. While these statements of faith aren't direct quotes from the Bible, they are drawn directly from Scripture. That is enough.
I do not follow Constantine's pagan-merged church
I also agree with you on this, except that it was more Theodosius I who made it really bad. Constantine, while good-intentioned, only opened the pathway. Theodosius I is the one who closed off all other pathways.
A true study of Church-history makes it clear that this was rampant at the time. I've taken seminary classes on this topic. My dad has his MDiv with a lot of church history stuff. From any good-faith biblical-historical scholar I've ever spoken with, the information we see is that as Christianity became a dominant religion across the Roman empire, other faith-systems were forced to follow the cultural tide. This meant, for example, that the priests in the temple of Mars (or in Greek countries: the priests in the temple of Zeus, or whoever), who had only ever been priests all their lives and knew nothing else but priest-ing, jumped on the Christian bandwagon and became priests in the Christian "temples" as well. They didn't magically know as much as the actual apostles, but the people were already used to following them, so they had influence.
This is why the councils had to happen in the first place. All the pagan teachings were blending with the teachings of actual Church (big C) leaders. In fact, some of the epistles talk about this blurring of doctrine and is, in some cases, why the epistles had to be written at all - to set the record straight. By the 300s AD it had gotten so bad that the Council at Nicea was one of the groups that tried to sort out the false pagan-blended from true doctrines.
By that point, there was a large push toward culturalization of Christian structure, which is where we get a lot of the liturgy, priests wearing robes, formalization of certain sacrements, etc. that just didn't exist from what we see in the Bible itself. So, I'm on-board with the notion that the ritualistic side of Christianity was a product of culturalization that really hit its stride in the days of Constantine through Theodosius I. There's a huge danger to that, which our modern-era congregational structure has been unable to escape. But I wouldn't say it's entirely bad. There's still value in it, if leveraged properly. It's not just the biblical necessity that poor ecclesiologists would have you believe.