You said, “The concept of confession of sin to a priest is nowhere taught in Scripture." First, the New Testament does not teach that there are to be priests in the New Covenant." That statement is completely false. First, let talk about the priests, when you see the word presbyters in the New Testament, in means priests. Jesus started the priesthood in the New Covenant. It is true that He is the High Priest. He offered Himself for our sins to the Father when He died on the cross. He is victim and priest at the same time. He instituted the priesthood in the Last Supper. He told the Apostles do this in memory of me. Do you know that was the first Mass? Jesus instituted together with the priesthood. He was victim and priest as priests offer sacrifices. The Mass represents Calvary. The priest offers Jesus as Victim as Jesus commanded them to do. That why the Mass is so awesome and a mystery. It is true worship of God, in Spirit and Truth as Jesus said in the Gospel. It was prophesized in the Old Testament. Read Malachi 1:11.
About confessions to the priests, Confession or also called Reconciliation is one of the seven Sacraments. Jesus instituted all of them and happened to be in the Bible, however as I shared before is not required to be. It is interesting that you said this,” Catholics do point to John 20:23”, after that silence. That verse said when you forgive men’s sins, they are forgiven, when you hold them bound, they are held bound. First, Jesus asked to forgive others with no limits. So how can we retain sins? If that applied to us, it will be a contradiction of the words of Jesus. Jesus will not contradict Himself, men do. Who has the authority to forgive and retain sins? Jesus gave to the Apostles. To forgive or retain, the priests need to know our sins, so we need to confess to them so if we are truly sorry to obtain forgiveness. Any questions let us know.
A lady I know, ask me the reasons to confess sins to a priest when Jesus is the only mediator. I told her that I may have the answer in this community which I do. The title of the discussion to share and specifically the second paragraph. I want to add if the person is not truly repented or did a bad confession, sins are retained. St. Father Pio did a lot of Confessions and God gave him a gift to know them. He would tell them. They were surprised for him to know and he retained their sins to come later when ready and truly repented and they need to tell the priest specially all mortal sins.