@india65 , thank you for your update. I'm sorry you are in so much pain. When we were communicating back in January, you talked about being in physical therapy. Are you still doing that now?

I am guessing that you don't do very much standing or walking because your pain limits you. The problem with sitting all the time (and this happens to me too) is that the hip flexor muscles get too tight, and those are pulling on your lumbar spine. Your pelvis can be pulled out of alignment which also adds pain, and you can have an upslip of your thigh in it's socket, or the illium (hip) bones can be inflared or have a slight twist. I speak from experience here because I have some laxity in my SI joints and when that happens, it causes low back pain for me. This is something a physical therapist would address. I know for myself, there is an easy maneuver to get my pelvis back in alignment. I have overly tight fascia on my left side through my rib cage and connecting to the hip bone as part of the thoracic outlet syndrome that I have. My PT works on that with myofascial release. When my hip flexors are too tight, I can lay on 2 small balls placed just inside the hip bones to release it. A PT can also use their hands to do that if you are laying on your back.

I realize that you have a fused lumbar spine, and I don't know what you can safely do. It is possible that other issues like pelvis alignment or tight scar tissue is creating a lot of pressure that is causing pain. I know we talked about myofacial release before, and I wanted to share this video with you explaining lumbosacral decompression. There is a lot of other information in that discussion and there is a provider search at http://mfrtherapists.com/

Video:
https://connect.mayoclinic.org/comment/247117/
Spine surgeons also do procedures to fuse the SI joints. That is something that you cannot undo, so it is worth exploring all other options before you consider that. I do know someone who had the SI fusion. She also has lumbar fusions that were done first, and had the SI fusion because she was still in pain. I think surgeons tend to think in terms of their procedures, and don't think a lot about scar tissue or tight fascial scar tissue caused by their surgery. They study on cadavers and it doesn't have the same properties or suppleness as live tissue. Your surgeon's comment about adjacent disc disease in your SI joint is a theory at this point. It could be out of alignment which also causes pain. Surgeons do explain that surgery can cause ongoing pain, but usually without an explanation of why that can happen. Let that be, without assigning a cause, and you will be better able to focus on getting better without having a road block in your mind.

This link explains a lot about pelvic dysfunction lumbar plexus compression syndrome. Your spine is fused, but your pelvis can move and change position. This link explains how that dysfunction mimics a lumbar spine problem by creating similar pain symptoms. It is a lot to read, but it will help you ask good questions.
https://mskneurology.com/identify-treat-lumbar-plexus-compression-syndrome-lpcs/
I have had good results with physical therapy so I tend to think of that first to try to solve a problem and it doesn't involve surgery or drugs. Does any of this sound like something you would consider before trying an ablation? Do you have any alignment issues with your ankles or knees? That too, can affect the pelvis and hips. Custom orthotics can help a lot from a podiatrist. I have them, and I just had impressions made for new ones because my ankle changed with an injury when I fractured it 2 years ago. My PT tells me that alignment is key to prevent wear and tear on knees that can lead to joint replacements.