Upon a quick search it looks like it’s done.
Revelation 22:18–19 18For I testify unto every man that heareth the words of the prophecy of this book, If any man shall add unto these things, God shall add unto him the plagues that are written in this book:
19 And if any man shall take away from the words of the book of this prophecy, God shall take away his part out of the book of life, and out of the holy city, and from the things which are written in this book.
Deuteronomy 4:2 2 Do not add to what I command you and do not subtract from it, but keep the commands of the Lord your God that I give you.
Proverbs 30:5–6 “Every word of God is flawless; he is a shield to those who take refuge in him. Do not add to his words, or he will rebuke you and prove you a liar.
Jesus quotes Genesis, Exodus, Psalms and Isaiah treating them as one authoritative Scripture despite them being individual books.
Paul cites “Scripture” while quoting multiple books
Peter calls Paul’s letters “Scripture” before any formal canon existed.
I agree that the books of Scripture were written individually and only later recognized as a canon. The Christian claim is that they share one divine Author. That’s why Scripture itself consistently warns against adding to God’s revealed word across covenants and genres. The unity isn’t based on later binding, but on a coherent, progressive revelation that Christians believe culminates in Christ and the apostolic witness.