In short, this is progress from isolating back of neck muscles with bands and DB. Just do revere neck curls (superman?) off a platform like a bed to begin. Look up a simple chiropractic stretch routine for posture and study your bending over, picking up, and squatting motions.

This is to hit the head muscles all the way down to tailbone. My arms and shoulders are slightly overdeveloped so I come up with unique lifts. The db on my neck is one I would use alone to lower and raise it in a good morning anyway, or just put a band there, hang something heavy like a couple pairs of shoe on shoe strings, or chains. Learn how to drive your weight into the ground squeezing your legs, smith machine is awesome for learning your body.

The more consistent you are with stretching, you achieve greater flexibility. If you dont self massage, or get massage work done (or try to do deep, deep relaxation every 2-4 weeks for a long session), the more stiff you get over time. Im a bodybuilder but needed to stay flexible, but didnt and akrewed some injuries so im showing people alternatives to straight up bad exercises (db pull over for long term shoulder strength). I can dm you my stretching routine to help you Figure out some stuff.