I am putting this out as an idea, as something to think about.
This assumes that the death and claimed resurrection of Jesus has soteriological significance. I am not seeking comments on the veracity of that position, nor with endeavouring to identify any Model that provides an "explanation".
I am simply asking, Did Jesus die in Nisan (March/April = "Easter") or would it have had proper soteriological significance if the events took place in Tishri (September/October)?
Is it possible - and I doubt there is any way of knowing - that the early Christians found it useful to incorporate existing Pagan "Spring" festivals, as this enabled them to differentiate themselves from the Jews' heavy focus on Tishri?
Yom Kipur/Day of Atonement, celebrated in Tishri, would seem to better represent the claimed soteriological significance of Jesus' death than a Passover lamb, which served only to mark those who were about to endure pursuit by Egypt, privations in the wilderness, and ongoing persecutions by powerful neighbours.