Keeping pelvis and legs in place?

I've recently been noticing that my pelvis tends to slide forward to the point where I'm kind of sitting on my tailbone like a shrimp which is causing some lower back pain and nerve pain in my leg. I don't have great core strength which is definitely part of the issue. My legs will also scoot/flop to the left and I end up sitting diagonally. Does anyone know where to start researching solutions? I currently have a leg strap that I use to keep my legs from flopping apart, I use a cross-body bag as a calf strap, and I have orthotic inserts in my shoes to keep my ankles in alignment.

I'm in discussion with my OT as well and we're both trying to find solutions but I don't think she's had any experience with paralysed clients before so all ideas so far have had to come from me. When I asked my other healthcare professionals who to talk to, they suggested contacting my rehab team but I never had rehab so that's not an option for me either. I'm in Canada, by the way.