Constantine The Great was not an Arian at all. In fact he was quite orthodox in his thought and to further this notion he is considered a saint in both the Catholic and Orthodox Churches.
Regarding Eusebius of Nicomedia’s confession of faith prior to the baptism of Constantine and during the First Ecumenical Council of 325, John Karmiris writes:
This answer is based on the following 3 resources I found:
A 2015 article Was Constantine the Great Baptized An Arian?
A 2012 paper The Spread Out of Arianism. A Critical Analysis of the Arian Heresy published in the International Journal of Orthodox Theology
A 2005 article How Arianism Almost Won by Christopher A. Hall published in Christian History Issue ...