I’ve used thin wire (think floral wire) for bendable crochet. I only used thin since the pieces were small but the wire gauge should be adjusted depending on the piece, I always curl the ends though
I have used floral wire and mechanics wire for posing and bending. You could crochet a piece slightly smaller than the neck size of your piece stuff it and insert it into the neck then stuff everything else normally. I have also used wooden dowels to stabilize the neck.
I go to dollar tree and buy hair curling rods. They come in different thicknesses, so it’s more customizable. I found these to be the best neck support for me. They also work on thicker pieces as a skeleton. But plant wire works well for that, too. Good luck with finding what works for you​