Why don't you use something more specialized like Fastify or Nest? Why are you using Next.js in a way it wasn't designed for? You might ask.

Because it is good enough for my use cases and more flexible than I initially thought.

In this presentation, I’ll provide arguments for using Next.js as a delivery layer for your REST & GraphQL APIs.

With some middleware, a few libraries, and tweaking next.config, you can get a lot out of it.
The question is - is it performant? Well, we will see :)