You’re exactly right to tie Psalm 2:7 and its New Testament echoes to kingship, mission, and divine appointment, rather than some eternal, ontological relationship within a Trinitarian framework. The phrase “Thou art my Son; this day have I begotten thee” (Psalm 2:7), quoted in Acts 13:33, Hebrews 1:5, and Hebrews 5:5, is consistently applied not to some timeless divine relationship, but to a moment in redemptive history—a "day” when God declares and reveals something new.
In Acts 13:33, Paul ties the “begotten” declaration directly to the resurrection of Jesus, not His eternal preexistence. In Hebrews 5:5, it’s linked to His high priestly role, and in Psalm 2, it’s clearly tied to royal enthronement—not divine biology. The term “begotten” here, especially in the ancient Hebrew and Greek context, is not about physical origin, but about investiture with authority—crowning a king, establishing a ruler, or publicly identifying a chosen one as God’s representative on earth.
This view aligns perfectly with how Sonship is presented throughout Scripture: as a role and identity rooted in the incarnation, ministry, death, resurrection, and exaltation of Jesus—not an eternal relationship between divine persons. When Jesus is called the “Son of God,” it’s declaring that this man, born of a virgin, filled with the Spirit, anointed without measure, and raised in power, is the visible image of the invisible God (Colossians 1:15), the appointed King, and the redeemer of the world.
So yes—Jesus’ Sonship was fundamentally about communicating His mission, not mapping a metaphysical relationship inside the Godhead. It’s a revelatory title, not a blueprint of divine anatomy. And that’s why Oneness theology holds that Sonship began with the incarnation—when the eternal God stepped into time, crowned Himself with human nature, and fulfilled the mission of salvation as the Son.