I had my COVID vaccine booster the day after IVIG, and was ill for over a week, conjunctivitis, full body rash, body pain, vomiting, diarrhea, was horrible. I had asked my doctor if it were OK to have them near and he said yes—HE WAS WRONG. I was on steroids for rash and conjunctivitis for over a week.
My hematologist said never have it near IVIG and wait mid-way, which I have done since the horrible reaction I had and have not had any issues since the timing.
Price, your intuition was spot on.