As an occupational therapist, I’d highly recommend either PT or OT depending on your goals. And, definitely try to find a therapist who has experience with scoliosis as therapists often focus on a certain population. Whenever I hear someone say that therapy was not helpful, I think, that either they just had a bad therapist (and there is a range of providers, just like doctors, some are good, some not very good) or that the therapist was not knowledgeable in your diagnosis and just provided general therapy activities.

PT would help with building or maintaining the best muscle strength that you can, maintain your range of motion, maintain mobility, and best positioning to minimize further curvature and/or improve organ function (fully inflating lungs, helping digestion, circulation).
An OT helps you find ways to accomplish your “occupation” - which is anything from house chores, self care, access to work tools, computer access, sports, hobbies, cooking, going out into the community successfully. They can also work on energy efficient adaptations. For example, with scoliosis you may become tired doing your laundry or chores. There are some things that would help reduce bending and lifting (like roll around carts) that can be adapted to your personal needs. Think about what activities are most important to you, and that should become part of the OTs goals. OTs also address positioning and mobility, usually related to your activity goals.
To get your insurance company to pay for therapy, they are always considering cost/benefit. If it will reduce their expenditures later on, they’re all for it. So when you explain why you need therapy, use terms the insurance company likes. I don’t mean lie, just help them understand it’s in their benefit to help you. “Therapy will help me keep independently mobile, therapy will help me care for myself longer by adapting self care routines or house chores. Therapy will help me stay employed longer because I can be mobile, maintain endurance, succeed at the work activities required by my employer. Therapy will help me stay independent and healthy lowering medical costs if I can safely exercise and/or engage in my hobby.”

Even a few appointments with a therapist can point you in the right direction and you follow through on your own. I think OTs and PTs are underutilized, but then I’m an OT and know what the outcomes could be.

(I was a school based OT and assistive technology specialist working with children in school. Scoliosis is not my area of training, so cannot suggest specific therapeutic solutions.)