We can look at one section of Isaiah and see what plays out in many other passages in the book and really throughout the Scriptures. There is a double aspect to the "day of the Lord." There is something both terrible and glorious. Justice and mercy are both revealed.
So "in that day" there is terrible judgment upon Zion (2:6-22), however, "in that day the Branch of the LORD shall be beautiful and glorious" (4:2 and see all of 4:2-6).
Jesus goes to the cross and the full wrath of God is terribly poured out upon Him. God's judgment is severe although righteous. This is because of His holiness, something we will see in Isaiah's vision in Chapter 6 as well.
Jesus goes to the cross and the mercy of God is displayed; His love is revealed in action. For God so loved the world that He gave His only begotten Son. God's merciful love is beautiful in its glory. This is because He is love and provides a Way of forgiveness and restoration.
Jesus comes to the earth and a remnant believe. He ascends to heaven and the church grows, many Jews and Gentiles come to faith. Pentecost-harvest is occurring. But these come out of old Jerusalem and the old Temple, the place which rejected God and His Son. There destruction falls and it is terrible (AD70). But there is the New Jerusalem now, the church, the bride of Christ, made ready by His Spirit in all of her glory.
In principle it is all finished. And yet we see the promises manifest over the course of this age as God works out His purposes of growing us up in our sanctification. It is in this process that we have the privilege of working out what God began - only to find out that we are really working out what God had finished.