For a mostly solo volunteer projet, supported by an individual who works as a high school teacher (so, not crazy-silicon-valley-rich), any such abusive costs are probably unsustainable.

Your figure of "<10k" that you consider as "minor" shows that you have no idea how people get by. They were kind enough to explain in advance how they would proceed if the complainant pursued in their misguided attack, how more agreeable do you want them to be without similar complaints drowning them out?