May I suggest you use a “raglan decrease” up to the neck. Lots of sweaters are raglan, with a “seam” from each side of the armpit up to the neckline. I like the idea of knitting this because there’s no sewing up seams for separate pieces afterwards (and I only just discovered what it was a couple of months ago!)
This link goes to a sweater pattern that’s knit from the bottom-up with raglan decreases (in the section “Yoke”). I think it would be fairly easy to apply to a poncho!