How do I compellingly advocate for my privacy with doctors and other healthcare professionals?

PurpleDime:

I inquired of my doctor’s receptionist about their privacy policy, and that’s exactly what she did

I would be surprised if receptionist knew any better. International law degree with emphasis on privacy in combination with setting up IT infrastructure is usually not a requirement for receptionist job title (nor for MD).

You mention GDPR; if you’re in EU, your doctor’s office should have a designated DPO (Data Protection Officer), and its contact is mandated to be listed. That is the person you should be contacting with your privacy concerns; not the receptionist or the medical doctor themselves (who [hopefully] have a different field of expertise).

PurpleDime:

What are the best arguments to make healthcare professionals care about privacy?

Failing solving the issues with the DPO; report them for violations to your country Supervisory Authority according to art.77 of the GDPR.

Then change the doctor, rinse & repeat until you get a good one – or you run out of doctors (in which case try not to get sick, or lower your privacy standards; whichever sounds less problematic).

Shall the SA also fail to address the privacy violations (or inform you of the resolution or progress within three months; you also have the “Right to an effective judicial remedy against a supervisory authority” pursuant to Art. 78 of the GDPR.