The shape, mobility and health of your spine is an important consideration when determining proper bike fit.
If your upper back/lower neck is too curved outward (kyphotic), there is a tendency towards more upper back pain/neck pain from the increased angle at the neck required to see the road. Decreasing the handlebar-saddle differential can help, with more or less reach determined by the impact to total curvature and shoulder-back integrity. Thoracic kyphosis tends to accompany a more flat/tight lumbar spine, so moving the saddle forward helps, too.
If the low back/booty is too J-lo-like (lordodic), the differential and reach need be adjusted to keep from excessive pelvic tilt. These clients are usually more limber, sometimes to excess. If the spine is overly mobile, power transfer from sacrum to pelvis to femur is altered under load, with symptoms ranging from head to toes.
It is difficult to discern if trunk angle limitations are from tight hamstrings, lumbar tension or pelvic morphology. While the symptoms and possible solutions for these conditions are many, typical accommodations reduced saddle height, reach and differential.
There are a number of other spine conditions that exceed DIY.
I recommend seeing a a qualified fitter if your back issues extend beyond the context of this post.