Anony Mouse...Even if the Christ/son was a mythic/mystic figure we can discuss the elements of the story. Paul's Midrashic style use of Genesis and Deuteronomy may be the key to how he concluded the Christ was hung on a tree in a spirit level above the firmament. In Galatians 3 he combines elements drawn from Deuteronomy "cursed is everyone hanged upon a tree" and the Genesis story of Isaac bearing upon himself the wood to be used for his own death as a sacrifice. Both of these pericopes inspired Jewish commentary and were linked through the use of the word "wood/tree". Note this comment from Wilcox, Max. 1977. “‘Upon the Tree’: Deut 21:22-23 in the New Testament.” Journal of Biblical Literature 96 (1): 85–99. https://www.jstor.org/stable/3265329
In Genesis 22 it is found in the plural ξύλα . . . for the wood of the burnt-offering and, more specifically, in 22:6 Abraham is depicted as taking the wood and loading it onto Isaac his son. Genesis Rabba comments on this, “like one who carries his cross . . . upon his shoulder.” A moment later Abraham is shown building an altar and setting out the wood upon it; he then binds Isaac and puts him “upon the altar, on top of the wood” . . . . In the NT model, in the fulness of time another comes to the place of sacrifice, carrying his “wood”/“cross” (cf. John 19:17), and is put upon it (cf. esp. 1 Pet 2:24 . . . ). We thus argue that behind the present context in Galatians 3 there is an earlier midrashic link between Gen 22:6-9 and Deut 21:22-23 by way of the common term … ξύλον …). That this has external confirmation we may see from (Ps.)-Tertullian, Adv. Iudaeos 10:6,
… Isaac, when led by his father as a victim, and himself bearing his own “wood,” was even at that period pointing to Christ’s death; conceded, as he was, as a victim by the Father; carrying, as he did, the “wood” of his own passion.
In short what is proposed is that the earliest Christians sprung from a Jewish faction that had integrated a son of man/son of God entity. Some of their writings are still available. Paul seems to be introducing the concept of the death of that entity through purely Midrashic means. IOW perceiving it from interpretation of passages in the Tanakh and related works. His contribution to the formation of Christianity as we know it was " Christ hung on a tree (crucified)".
It's not hard then to imagine the link to the Roman practice of crucifixion on a cross. It cannot be precisely identified who first made this link as the work of Paul have been heavily redacted and selected and very little else has survived from this period. However, by the time the narrative Gospels came along the link to the Roman practice and period was fully established. Therefore, as Wells finally concluded it would be possible an actual historical person's execution became integrated into the Christ composition. This person/s may have been someone from an earlier period such as the Teacher of Righteousness who was killed by the religious leadership or someone like the Jesus the prophet who was supposedly killed with a Roman catapult in Josephus or one of the many would be reformers and messiahs.