The council of Nicea historically speaking is listed as the first ecumentical council of the church held in order to contradict and refute Arianism. This is conceded by Roman Catholicism, Eastern Orthodox and Protestantism.
Saying it is wrong does not make it so. Appeal was made specifically to Scripture confirm what they had discussed. James used Amos 9 as his evidence for the conclusion in the matter. They all discussed the matter specifically Peter and Paul to give insight to what was going on in their ministry. Scripture is supreme as I said. That was displayed there contextually and exegetically. Issues were discussed and opinions stated and then a conclusion based on Scripture which confirmed their views. The Law and the Prophets. was their Scripture as individual NT books were being written and added to the rule of faith as inspired Scripture. Scripture is Scripture in it’s nature and quality. You appear to try and set forth a contrast between the Old and New Testament when that is unnecessary to do.
They get their " doctrine " from Scripture right ? Are elders invent unknown doctrines that no one else knows about ? The word " doctrine " is stated in that verse right ? Didakalia is instruction or doctrine of which the people of God are to learn from Elders who teach them.The elders get this instruction or doctrine from Scripture. Paul used Scripture as proof for his view of elders of being suported in those verses. Thus Scripture is clearly seen as the rule of faith there being appealed to.
Strawman. I dont hold Scripture is the " sole rule " as you stated. It is the sole infallible rule of faith. I do see lesser rules of faith such as creeds and tradition and councils. Try and deal with what I believe and not resort to strawmen please.
Jesus used Scripture to combat the temptations and the lies of Satan. Jesus appealed to Scripture in this. Jesus by his exampled followed God’s Word in overcoming temptation from Satan. " It is written " shows His direct reply to Satan from Scripture itself. No appeal is made to a so called unwritten doctrinal oral tradition but to the Old Testament itself. Jesus taught Scripture as our rule of faith.
During that time they had both the oral word of God and the written word of God. Your question appears to create a strawman or misunderstanding of Sola Scriptura. What is claimed is now since the time of the apostles we have only God’s Word contained in written form that being inspired Scripture. We are not presently being given new divine revelation outside of Scripture. In other words, we have no prophets or apostles today. Basically God’s Word is strictly speaking found in His written word the Scriptures now.
They only confirmed what was already in Scripture. They did not invent new or unknown doctrines. The church is fallible otherwise you cant explain to me the heretical Arian Councils during the time of Nicea. We have the sure word in Scripture. Scripture is infallible.

God allowed fallible men to exist since Adam and Eve.

The promise of salvation is based on the fact that God cannot lie. Since Scripture is God’s written word it by it’s nature has the authority of God of which there is no higher authority.