I'm so sorry that your pup has this. My Rory has SM and began showing mild signs at about 4 months. We pursued surgery at 20 months and he is doing well post=op, but he was not showing very severe signs when we decided to do the surgery. He is not 100% better, and may be worse. Hard to say because we also started him on medication post-op that has really really helped. If he misses a dose, then he gets very irritated and scratchy. Right now he is convinced something is biting his back feet and desperately trying to get away from wherever the evil feet biting monsters are.... Even though he just got his medicine 4 hours ago...
Basically - the surgery is not 100%. You need to know this. It is not a cure. It can halt progression, but not always. I am not sure it halted progression for Rory. But even so, I know it was the best choice for him to possibly lead a normal life. He is also on Gabapentin and Lasix (frusemide) and this definitely helps.
If your dog is very severely affected, then your options are probably to pursue surgery and keep your fingers crossed, or try to keep her comfortable for as long as possible... If surgery is an option for your family (it is a big financial commitment) then I would say go for it. What have you got to lose? For me - I knew that even if the surgery didn't work, I had to try it. I had to give him that chance. I do not regret it for a moment.
I would, however, see about travelling to a more experienced vet. Are you anywhere near London?? Dr. Clare Rusbridge is the premier neurologist/researcher on this disease in the UK. Myself, I flew about 4 hours away to have the surgery performed by a neurologist who I felt very comfortable with and who had performed the most surgeries that I was aware of. Also - Dr. Rusbridge may be trying the new mesh placement to prevent scar tissue?? I think?
Please feel free to PM me if you have more questions about any of this.
Dogs are not our whole life, but they make our lives whole.