Perhaps history is difficult to evaluate fully, as so much time has passed coupled with the fact that we are limited by our access to original sources and affected by historian bias of whomever we read.
With this "asterisk" in view as a plea for humility on all sides, let me offer this piece as an attempt to bridge the gap in this conversation.
It is helpful to note at least 3 realities bearing on this topic:
1. Jesus Christ began His Church from the 12 disciples and the 120 active followers after His death. The Church era officially launched at Pentecost with thousands coming under the umbrella of the true Church of Jesus Christ and rapidly spread from initially Jewish believers to Gentile believers from all over Asia Minor. While Peter and James exercised initial prominence of leadership among the Jewish Christians, Paul was the primary spiritual father of the newly planted churches throughout Macedonia. By the close of the the New Testament era, there were tens of thousands of believers in many hundreds of locations throughout the Roman empire. Over the next 200-300 years Christians were regularly persecuted (systematically first by Jews and then by pagan Romans) while also continuing to expand exponentially. Early Christianity (pre-Constantine) was not highly theological in terms of forming tight doctrinal statements to pass the test of time. This requires the deep reflection opportunity of peacetime, and early Christianity was forged on the run. Yet reading writings by Justin Martyr, Turtullian, and an assortment of other early Fathers shows the formative seeds and beginnings of deep roots than Athanasius, Augustine, Gregory of Nyssa, Basil, Chrysostom, and many others would continue to develop and hone.
The main point that everyone needs to grasp is this: True Christianity did not begin with Constantine, nor was it formed by Constantine. The Church of Jesus Christ is His Church, empowered by the Holy Spirit that was represented by many thousands of true saints prior to Constantine, flowing from the group leadership of multiple apostles, and steadily developing in core doctrines over several centuries, forged by persecution and debate. Constantine had a meaningful and far reaching role in making Christianity the State Religion, officially stopping formal persecution, calling councils, and establishing unholy alliances between emperors and religious leaders. But he had little affect on true spiritual Christianity.
2. The Councils and Creedal formation processes were messy and imperfect. It is a sophomoric error at one end for anyone to continue to perpetrate the silliness that Constantine and the Nicea council gave us the Bible. In truth, all the inspired "books" of Scripture came to be recognized progressively by Christians via usage prior to Nicea. The full Old Testament was established and in place by the time of Jesus. The 4 gospels and core letters of Paul (including Hebrews, as it was frequently thought to have been directly his or associated with him), Peter, and Timothy had been long recognized as Scripture. It has been long and indisputably established that the core of authentic Scriptures had been initially established by (the heretic) Marcion in the 2nd century. There were a few disputed letters (2 Thessalonians being one), but the point is that Nicea did not establish the Bible so much as merely affirm what the church process had already affirmed in regular usage. Their role was one of official codification.
It is similarly an error at the other end to take the then-contemporary conclusions as final gospel truth for all time.
3. The primary issue at Nicea which personally involved Constantine concerned the understanding of the Divinity of Jesus. Constantine himself went back and forth over the years (largely according to his personaly associations with Arians) between supporting Arius' view of Jesus being a the first derived being from God the Father -- as a means of protecting the doctrine of "one true God"; and supporting Athanasius' view of Jesus being of the same substance/essence as God - being in some form of a dual-unity; thus preserving the doctrine of "no other gods". The question was a valid one to hammer out. Interestingly as the drama unfolded... the Council began with a decidedly Arian bent. Basil proved to be politically instrumental in uniting the semi-Arians (who maintained a distinction between the Father and the Son) with the Nicene party (who emphasized their essential unity) against the Arians. This argument and political base eventually won the day... for awhile...
This victory however was short-lived. One season's court theologian became next season's heretic. Athanasius, the most famous Nicene theologian, was banished and re-established multiple times during his lifetime. Future councils continued to fight this issue of confirming or denying the conclusions of Nicea for the next 200 years. Eventually, Basil's original Nicea wording won the day, and has become considered "orthodox", but it had always been a politicized venture. I'd be very careful to not use "orthodox" as a club against competing viewpoints that were themselves labeled orthodox for a season. Those who believe that God infallibly sanctioned the processes and conclusions would come to find God changing His mind multiple times over several centuries!
The point here is that we are far too prone to have glossy views of history. God works through history, but sin and Satan are working as well.
This cumulative combination of realities makes it quite easy to conclude: While God utilized early Catholicism, including the process of some of the councils and some of the resultant creeds ... however valuable they are, they are certainly not authoritative. God saw fit to advance His Church through their usage, however imperfect. Meanwhile, we still have access to the perfection of His Word in flesh (Jesus), in Spirit (internal witness of the Holy Spirit), and most reliably for interpretation via His written Scriptures (the Bible of both testaments).
The fact that God utilized the early Catholic churches and creedal processes do not make them authoritative. They have ongoing value only to the degree that they conform to the ongoing authority of God's written Word and truth in Jesus.
Meanwhile, the syncretism between paganism and Constantinian catholicism was real. They did, in fact, take pagan holidays and attempt to renovate them, or more nobly to "redeem" them into Christian holidays. This reality is this attempt was likely neither horrible nor idyllic. I tend to think it was a noble mistake. Better likely to have established a celebration of Christ's birth and death and resurrection completely apart from associations with the Roman pagan calendar and to simply skip the ritual trappings.
Yet anyone who uses the only-partly redeemed paganism reality as justification to re-embrace pure paganism as somehow more valid and lasting is a fool. There are no local deities, only local demons. There is one true God over all: Yahweh in 3 persons -- the Father, Son, and Holy Spirit. Eternal Life with God is through the Son, our one mediator -- Jesus Christ.