We have not gone through similar, so can't comment too directly, only on your statement "...apparently in true PANDAS cases it [OCD] accelerates anxiety and vice-versa?"
Well, I would say that OCD, if it starts more slowly in a PANS/PANDAS child, is somewhat calming at first, when it is not otherwise debilitating or noticed by others too much, because there is some satisfaction derived from "repeating it right" when they finally get there. But when it grows much more significant later (or for any person with severe OCD) the sufferer fairly quickly becomes exasperated with how debilitating it is, and sometimes satisfaction is eluded, and so of course that creates plenty of anxiety. So I think your statement is fully true for those with very quick onset of PANS/PANDAS, and eventually true for all.
If you can get and afford the ivig, the potential benefits probably outweigh the risks (in my mind).