If pain management/therapy/chiropractor doesn't sucess after spine fusion surgery, what is next?

SCS. Spinal cord stimulation may be bery helpful. It may however not be either helpful or tolerable. A two to three day trial can allow for a decision to be made.
Psychotherapy. Chronic pain pts usually have some depression.Mental health is important in the recovery. If condition not improve with pain management and chiropractor, i would think that psychotherapy is strongly suggested to look into .
Hard to answer. "pain management" is a broad term. This could range from meds only to comprehensive management. Not sure what has been done or tried. In my practice the last resort is ofter spinal cord stimulation or possibly a pain pump.