>> They load Google Tag Manager, Google Retargeting Pixel, Segment, Facebook Connect, and SmartLook (which looks like some kind of analytics package).
>> This should not be controversial
This is tricky, as handling a person asking for a medical advice is a rather special thing, compared to a person asking to buy a vacuum cleaner.
The thing is, all of your medical info will go to all of the above trackers, and surely they will sell it to whoever wants to pay for that. Insurance companies included.
you can set up your corp once, then present it as "doing-business-as" (DBA), depending on the jurisdiction, many times. I am not convinced that the above is good advice.
In fact, talking to a lawyer is probably better idea than picking up ideas from some blog.
This is really about having SSO, not about Octa or Google.