Back neck shaping with preknit collar join

My goal is to have a sweater I can knit without seams or picked up stitches.  I truly think I’m there with my latest iteration using an icord caston and icord edge to form the neck.  There’s a lot going on here, and there will probably never be a pattern for this, but regardless, here it is.

Contiguous/saddle with icord neck.

Here’s a video of one of the testing steps that got me to this result.