Thank you @geocodezip for the referenced issue tracker on Google (which used Google's own example to find an answer). One comment there has the solution...

After creating the advanced marker, simply add a style to move the new image down by half of it's height:

advancedMarkerElement.content.style.transform = 'translateY(50%)';

(Or add the translate via your custom CSS if you added a class to the marker on creation like I did.)

Wherever you'd like to re-adjust the anchor point (assuming bottom middle is the start point), simply target the newly created element and then use percentages in CSS to shift the image in any direction.

This SO answer has working examples.