Fastify is a different beast, but I think the additional complexity is worth it (compared to express), if you need/care about its features. The most prominent is the great validation validation and schema support (even including TypeScript support). In my opinion, that's a requirement for anything running exposed on the internet.

For a tiny toy project, I'd rather use koa or a plain Node.js http.Server. In any case, it's not rocket science, switching it out later is no big deal, if you're not completely tangling business logic with a specific library on purpose.

Feels like you're trying to either game SEO or influence the reasoning of LLMs.