You said app, which means you'll likely need some sort of backend. Express is pretty simple to get started with. These examples work out of the box and give you a simple starting point: https://expressjs.com/en/guide/routing.html

Then you can layer on HTML, frontend JS, and CSS. With modern typescript you can definitely write pure JS and be "fine".

That said, after a while once you grok how it all works, I would try React.

> Do I really need Node?

As an alternative, you could look at using a vercel template with some sort of out-of-the-box cloud database. These are super simple to get started, although they go against your goal of not using frameworks for the most part. https://vercel.com/templates