What to do to improve my neck lines? (Photo)

If you didn't have any improvement from Morpheus8 you could try FaceTite. This is a minimally invasive RF treatment and requires no general anesthesia. If that doesn't help you will likely need to consider a revision for further improvement.

When there is too much loose skin, an isolated necklift just won't do. As you have learned, skin tightening devices have their limitations.
At this point I would consider a face lift in addition to a necklift. You need to pull up the slack between the clavicles and the midface with upward traction on the SMAs/platysma, then re-drape and remove the excess skin of the neck/lower face.

I would recommend a deep plane face/neck lift. This would best address the deep structures below the platysma muscle in the neck and also treat the loose platysmal fibers in the neck that are causing the vertical bands. The skin will also be released and re-draped after proper lifting of the face and tightening of the neck. You need to find an American Board of Plastic Surgery certified surgeon; ideally, this surgeon has also completed an aesthetic fellowship accredited by the Aesthetic Society.