Let's establish from the bible the origin of Jesus. This part is important - we have 4 different gospel accounts that together provide a sound understanding in need of no interpretation.
…it seemed fitting to me as well, having investigated everything carefully from the beginning, to write it out for you in an orderly sequence, most excellent Theophilus; 4 so that you may know the exact truth about the things you have been taught Luke 1:3
the angel said to her, “Do not be afraid, Mary, for you have found favour with God. 31 And behold, you will conceive in your womb and give birth to a son, and you shall name him Jesus Luke 1:30
He will be very great and will be called the Son of the Most High. The Lord God will give him the throne of his ancestor David. v32
The record of the genealogy of Jesus the Messiah, the son of David, the son of Abraham: Matt 1:1
Now the birth of Jesus the Messiah was as follows: when his mother Mary… v18
We have the beginning of ‘Jesus’, the Christ. A descendant of David and Abraham. No one is talking about Jesus being God, no one speaks of his pre-existence as some eternal being – an ‘exact truth’, we’re told or ‘perfectly’, ‘diligently’, ‘accurately’ as other translations render it. A baby boy born of Mary, named Jesus, the son of God.
Gal 4:4 But when the fullness of the time came, God sent His son, born of a woman, born under the Law.
From this humble beginning we have the rest of the NT which expands on the life of this Jesus, who grew to be a man, who lived an extraordinary life without sin and died as prophecy foretold, was raised back to life - not just physical life but eternal, spirit life, by God and exalted to the right hand of his God and Father!
Did David have God as his descendant, did Mary birth God?? Well, no one writes about this at all – nothing. Just a normal baby, born holy with a predestined future involving some hideous suffering and trial – not because he deserved it, but because God had arranged for him to pay for all sin with his perfect life. A model of sacrifice not needed to be done often as the Jews/Israelites were used to, but once only and for all creation/mankind.
This is the simple truth - no incarnation, no pre-existence, no God becoming a man. All these things, sadly, are made up to present another gospel about another Jesus who is not described in the bible at all - save a few 'proof-texts' that can be forced to infer something accommodating a traditional theology.
Perhaps those believing in a pre-existing Jesus have never read these Gospel accounts as the clear evidence offered says nothing about the ideas many seem to hold dear.
Even in Jesus' own words, he offered,
But now you seek to kill me, a man who has spoken to you the truth that I heard from God. John 8:40
A man, a man who could die, (without having to make up a fanciful two-natured Jesus) a man who could be tempted, made like us in every respect Heb 2:17, a man who was "mastered by death" Rom 6:9. Unfortunately, a God/man Jesus does not fit very well with any of these truths and we have to start making up stuff and mis-appropriating verses to counter the bible's revelation of the true Jesus.
The biblical basis for Jesus beginning to actually exist at his conception and birth is simply what the bible expresses quite insistently. Why is there a burning desire to perpetuate another story that disagrees with the biblical one? Not only disagrees, but resists and denies!
The biblical basis is also based on the absolute silence of Jesus before the Gospel accounts. Those that immediately jump on John 1 seem to have trouble reading the 'word' (logos) that John was inspired to write, and read-in Jesus instead. It makes no sense to insert Jesus where he does not belong. John writes the "logos became flesh", which happened ~2000 years ago so Jesus cannot have been, 'in the beginning'.
The idea of a 'notional existence' is not a biblical term. Simply it refers to the plan God always had in place, the son He would arrange when the time was right. This was always going to be the way it is. God was sending a Saviour before the world began. Did he already exist? Not according to the bible, no. The plan is revealed as prophecy all of which came true as Jesus lived it out - often drawing attention to this as he did.
The term notional represents the truth that what God has said will be - already IS, even though it isn't yet. IOW, God's plans will not be thwarted - what He says He will do will be done and is as good as done once He says so. The only difference is time between the decree and the fulfilment of such.
Jesus was part of God's plan from the beginning but only 'in these last days' (Heb 1), has he been the actual manifestation of God's plan - the 'logos made flesh' ~2000 years ago.
Rejoice greatly, daughter of Zion! Shout in triumph, daughter of Jerusalem! Behold, your king is coming to you; He is righteous and endowed with salvation, Humble, and mounted on a donkey, Even on a colt, the foal of a donkey. Zech 9:9
“For the time is coming,” says the LORD, “when I will raise up a righteous descendant from King David’s line. He will be a King who rules with wisdom. He will do what is just and right throughout the land. (Ps 110, Ps 22, Is 53 also)