Combining Static Sites + Client Side Rendering (React, Gatsby)

I want to build a web app with React.

When users visit the site, they will see a Landing Page, can go to a Pricing Page, an About page, a Blog. etc. They can also Sign Up or Log in and then there's the actual app. I would like render certain pages (Landing,Pricing,About,Blog) statically, but would like to leave everything behind the SignUp/Login-Wall client-side rendered.

(First, because it cannot be static, since this is dynamic content. And also, because I do not care about SEO here anyways, so a major reason for next.js falls away, since the app is behind a SignUp/Login Wall anyways.)

Questions: First of all: Does this make sense? And secondly: How could I implement something like this? I haven't found anything online! Is this unheard of? I would like to use Gatsby.js for my static content, but I am not sure how to bring the client-side-rendered bit into the mix. I have worked with create-react-app before, which does client-side-rendering, - but I am not sure how I would go about the implementation?

I will try to explain the process behind jamstack-hackathon-starter (which @ksav commented).

It is a starter template for gatsby that allows you to save static pages in-
conjunction with dynamic pages (client-side react app) - a "Hybrid Gatbsy App".

Manual Steps:
1. Create a folder src/app which will contain your client-side react app.
2. Create a file in src/pages called app.js, with the following contents:

// I'm using create-react-app's structure
import App from '../app/src/App' // make sure this path corresponds to your path

export default App

3. Now install gatsby-plugin-create-client-paths:
   npm install --save gatsby-plugin-create-client-paths
4. Configure it by adding it to gatsby-config.js like so:

 plugins: [
 {
      resolve: `gatsby-plugin-create-client-paths`,
      options: { prefixes: [`/app/*`] },
 },
 ...

5. This will result in everything within /app to only be rendered in the browser (ie client-side).
6. Go to your browser after building (gatsby develop) and check /app