On a recommendation from a friend I filed a complaint online at https://ag.ny.gov/internet/online-forms. I wrote up the situation and attached a pdf copy of the support interaction I'd had with Google.

It wasn't quick, but it did seem to work. Here's the timeline:

- May 2018: I contact Google about being unable to export my accounting data from last month

- November 2018: after nothing but "thank you for your patience" emails, I complain to the AG

- early February 2019: I get a letter in the mail from the AG saying they'd forwarded the complaint to Google

- late February 2019: Google finally gives me my data [edit: I'd had April originally, but that was a separate request].