Hi. I don't have any experience with SPS myself but I did read about it after seeing your post. There are many positive reports about the effectiveness of IVIG for SPS... for example, in one study it reports: "On the basis of a double-blind placebo-controlled trial conducted in SPS patients with high anti-GAD antibodies, IVIg was shown to be effective resulting in improvement of stiffness and heightened sensitivity scores and increasing the patients' ability to carry out daily activities."
I wasn't able to include the link to the study here, but if you google SPS and IVIG it will come up along with other positive examples like this.

Hopefully there is someone on this forum who has had experience with it
who will respond to you soon.
I am very hopeful for you. Let us know how it works for you.