Hi Sven,
Is there any indication that suggests your SFN is autoimmune related ?
As far as I understand IVIG would help only when the SFN is caused by immunological problems.
Maybe you could ask your neuro to try a short course of steroids, it might help to estimate whether immunological treatments, such as IVIG, could work.
If you are paying by yourself for an off label treatment you might want to consider Rituximab instead of IVIG. IVIG is the conventional treatment for autoimmune related SFN and there is only very little and anecdotal research regarding Rituximab in these cases. Rituximab is used in the treatment of autoimmune disorders (and in Lymphoma), it is expensive, but much less than IVIG and the frequency of the infusions is much lower.
Usually the initial dosage is 2 infusions 2 weeks apart each one 1000mg. It starts affecting within 2-3 months and if it works it is usually repeated only after 6 months. I don’t know the costs in USA, here 2x1000mg costs about 7K.
I got it about 7 months ago, it helped tremendously and I’m about to get it again soon. But my symptoms are much different than yours and it seems that I have got an autoimmune disorder.

Do you have just SFN (and, yes, “just” SFN is more then enough..) or other health problems ? Usually in autoimmune disorders there are more problems.

Have you ruled out other causes such as diabetics, pre diabetics, side effect of medications as antibiotics, vaccines, nutritional deficiencies etc. ?

Have you tried taking supplements, keeping on a healthy nutrition, physical activity, reducing stress, etc. ?
I’m taking Vit B12, C, D, E, CoQ10, and Calcium and keeping on a healthy (vegan) nutrition and it helped the SFN symptoms.

I hope you would find a way to be able to return to work!