Paul tells us WHEN AND HOW Jesus became "the only begotten son" of Jehovah/YHVH God;
Acts 13:29-35 And when they had fulfilled all that was written of him, they took him down from the tree, and laid him in a sepulchre.
30 But God raised him from the dead:
31 And he was seen many days of them which came up with him from Galilee to Jerusalem, who are his witnesses unto the people.
32 And we declare unto you glad tidings, how that the promise which was made unto the fathers,
33 God hath fulfilled the same unto us their children, in that He hath raised up Jesus again; as it is also written in the second psalm, Thou art My Son, this day have I begotten thee.
34 And as concerning that He raised him up from the dead, now no more to return to corruption, he said on this wise, I will give you the sure mercies of David.
35 Wherefore he saith also in another psalm, Thou shalt not suffer Thine holy one to see corruption.
1 Cor 15:20 But now is Christ risen from the dead, and become the firstfruits of them that slept.
Col 1:18 And he is the head of the body, the church: who is the beginning, the firstborn from the dead; that in all things he might have the preeminence.
Rom 8:29 For whom he did foreknow, he also did predestinate to be conformed to the image of his Son, that he might be the firstborn among many brethren.
Jesus IS "the only begotten son" ........ so far. When he returns to this earth, then ALL the righteous will be "begotten:
1 Peter 1:3 Blessed be the God and Father of our Lord Jesus Christ, which according to his abundant mercy hath begotten us again unto a lively hope by the resurrection of Jesus Christ from the dead,
The word "begotten" typically is used in the sense of conception.
Matthew 1:20-21 KJV
(20) But while he thought on these things, behold, the angel of the Lord appeared unto him in a dream, saying, Joseph, thou son of David, fear not to take unto thee Mary thy wife:
for that which is conceived in her is of the Holy Ghost.
(21) And she shall bring forth a son, and thou shalt call his name JESUS: for he shall save his people from their sins.
The sense of "begotten meaning "raised from the dead' doesn't seem to fit the sense of what is being described here in Hebrews. Jesus referred to God as His Father years before he was raised from the dead. Yet all three events in Hebrews would be satisfied with his conception by the Holy Spirit, if they are being spoken of in parallel.
Hebrews 1:5-6 KJV
(5) For unto which of the angels said he at any time,
Thou art my Son, this day have I begotten thee? And again, I will be to him a Father, and he shall be to me a Son?
(6) And again,
when he bringeth in the firstbegotten into the world, he saith, And let all the angels of God worship him.
So, Acts 13:33, are you sure that (the way you suggested, resurrection) is the only way it can be reasonably read?
Acts 13:33 KJV
(33) God hath fulfilled the same unto us their children, in that he hath raised up Jesus again; as it is also written in the second psalm,
Thou art my Son, this day have I begotten thee.
It also could be understood in that the fulfillment of the Son is the promise made unto their fathers, and that in raising up Jesus it affirms that Jesus was indeed that Son of God.
Consider also,
Matthew 3:16-17 KJV
(16) And Jesus, when he was baptized, went up straightway out of the water: and, lo, the heavens were opened unto him, and he saw the Spirit of God descending like a dove, and lighting upon him:
(17) And lo a voice from heaven, saying,
This is my beloved Son, in whom I am well pleased.
How would Jesus first be an son of God (unbegotten) and then begotten after the fact?
In your favor, I do see how "begotten" is used in 1 John 5:1, 18, also Revelation 1:5. But here (following) is what throws the monkey wrench into play, concerning what is likely meant by "only begotten son of God."
1 John 4:9 KJV
(9) In this was manifested the love of God toward us,
because that God sent his only begotten Son into the world, that we might live through him.
The sense of that passage doesn't quite match if Jesus was sent into the world without being his only begotten Son, and was only begotten after the fact of his being sent. I suggest that "begotten" might be used in more than one sense in these scriptures, and that "only begotten Son" should not be confused with "first begotten of the dead" and "begotten of God." I don't think those are meant to be the same.