What will Christ's handing everything over to the Triune God accomplish?
It will accomplish the consummation of God's whole eternal purpose. After Christ's millennial rule, all things are delivered to the Triune God, bringing creation from eternity past into the new eternal state—new heaven, new earth—untouched by Satan or defilement, with God fully consummate and even the Lord Jesus under the Godhead's power.
Les explains that after the thousand years of Christ as King of Kings and His absolute rule are fulfilled, Christ will give everything up to the Triune God. This is the consummation of the whole plan that began in eternity past when creation was first called into being, continuing until the new eternal state. The outcome is a new heaven, new earth, new everything, with nothing touched by Satan or the things that defile. Under those circumstances, God will be consummate, and even the Lord Jesus will be under the power of that Godhead. Les then moves to verse 29, acknowledging it is a tough and confusing statement about those baptized for the dead, which he refuses to skip.
And when all things shall be subdued unto him, then shall the Son also himself be subject unto him that put all things under him, that God may be all in all.