I was diagnosed with Dermatomyositis an autoimmune disease in January of 2021. I have been getting Gammagard IVIG treatment for over two and a half years, every four weeks. Like you I was very scared, but IVIG has helped me immensely, it has keep me stable. I have, skin, lung, and muscle involvement with this disease.
The thing that helped the most when I first started IVIG, were two Private support groups on Facebook. Intravenous Immunoglobulin (IVIG) Support Group and the other one IVIG and SCIG Education and Discussion Board .
From these groups, I learned what to expect, and best way to prepare for IVIG treatment. Also if I have questions or concerns, the group is there to help, and also to support you. I have learned so much from these groups, and the support is amazing because they understand what you are dealing with.

Hydration before and after infusion is very important, as well as your infusion rate, because if your infusion rate is too high your side effects can be worse.
I also get Tylenol and Benadryl before my infusion.
Hope this helps, Good Luck to You.
Kathryn