Yes, there is only one God who is indivisible. That agrees with the doctrine of the Trinity.The idea that the Word is “eternally begotten” as the “Son of God” can be challenged by emphasizing that Scripture teaches a singular, indivisible God
3. And the catholic faith is this: That we worship one God in Trinity, and Trinity in Unity;
4. Neither confounding the persons nor dividing the substance.
5. For there is one person of the Father, another of the Son, and another of the Holy Spirit.
6. But the Godhead of the Father, of the Son, and of the Holy Spirit is all one, the glory equal, the majesty coeternal.
7. Such as the Father is, such is the Son, and such is the Holy Spirit.
8. The Father uncreated, the Son uncreated, and the Holy Spirit uncreated.
9. The Father incomprehensible, the Son incomprehensible, and the Holy Spirit incomprehensible.
10. The Father eternal, the Son eternal, and the Holy Spirit eternal.
11.And yet they are not three eternals but one eternal.
12. As also there are not three uncreated nor three incomprehensible, but one uncreated and one incomprehensible.
13. So likewise the Father is almighty, the Son almighty, and the Holy Spirit almighty.
14. And yet they are not three almighties, but one almighty.
15. So the Father is God, the Son is God, and the Holy Spirit is God;
16. And yet they are not three Gods, but one God.
17. So likewise the Father is Lord, the Son Lord, and the Holy Spirit Lord;
18. And yet they are not three Lords but one Lord.
19. For like as we are compelled by the Christian verity to acknowledge every Person by himself to be God and Lord;
20. So are we forbidden by the catholic religion to say; There are three Gods or three Lords.
21. The Father is made of none, neither created nor begotten.
22. The Son is of the Father alone; not made nor created, but begotten.
23. The Holy Spirit is of the Father and of the Son; neither made, nor created, nor begotten,but proceeding.
24. So there is one Father, not three Fathers; one Son, not three Sons; one Holy Spirit, not three Holy Spirits.
25. And in this Trinity none is afore or after another; none is greater or less than another.
26. But the whole three persons are coeternal,and coequal.
27. So that in all things, as aforesaid, the Unity in Trinity and the Trinity in Unity is to be worshipped.
28. He therefore that will be saved must thus think of the Trinity.
https://www.ccel.org/creeds/athanasian.creed.html
And, yet, as I pointed out, having Christ's Sonship begin at the incarnation is problematic. Jesus said that the Father loved him before the foundation of the world. Who, exactly, was it that the Father loved prior to creation?who became incarnate as the Son of God in time, not in eternity. The concept of “Sonship” can be understood as a role or office that began with the incarnation—when the eternal Spirit of God (the Father) was manifest in the flesh as Jesus Christ, the Son of God. The title “Son” thus pertains specifically to God’s redemptive work and His incarnation, rather than to an eternal, distinct person within the Godhead.
Here is the main problem with such an idea: if the Word is merely God's "divine reason and plan," if the Word has no "personhood" to begin with, then Jesus is merely a creation of God, no different from any other creature. Right? The divine nature in Jesus becomes no different than God saying '"Let there be light," and there was light.' Jesus could not even be the Father come in the flesh, since it was the non-person Word that became flesh.In John 1:1, “the Word” (Greek: Logos) is not a separate, eternally begotten Son but the self-expression of God, His divine reason and plan.
So, God loved "His plan and purpose" before the foundation of the world? Does that really make sense? Again, the idea of a God who is love is an impossibility in Oneness theology, as it is in all unitarian theologies. It makes God less than he is.When John writes, “the Word was made flesh, and dwelt among us” (John 1:14), he is describing the moment when God’s eternal Word—His plan and purpose—became a reality in the man, Jesus Christ. The term “Son” is applied only to Jesus’ role in the flesh, not to the Logos in eternity past. Therefore, the Son was not eternally begotten but “begotten” in the sense of a unique, historical act of God in the incarnation. I would maintain that Scripture affirms the Son’s existence within the framework of time, emphasizing that Jesus Christ, as God manifest in flesh, fully reveals the Father rather than existing as a second, eternally begotten person.
The Sonship of Jesus only "within the framework of time" simply does not fit the biblical evidence.