“The site is a year old, perfectly functional, and I had already submitted a sitemap in Search Console, and yet even though Google has a sitemap showing 9 pages, Google had only indexed the home page.”
DuckDuckGo indexed the whole site. Google took the home page and ignored the rest. The only fix that worked was forcing pages one at a time through Search Console’s URL Inspection. The folk remedies are installing Google Analytics and emailing the link from Gmail. Google used to index every site whether you wanted it or not, and now it waits for you to sign up.