In my experience, there’s no casual mode within React. You need to be all-in, keeping up with the ecosystem, or else your knowledge evaporates.
I think Dave is right. At this point, it’s possible to be a React developer exclusively.
React is an ecosystem. I feel like it’s a disservice to anyone trying to learn to diminish all that React entails. React shows up on the scene with Babel, Webpack, and JSX (which each have their own learning curve) then quickly branches out into technologies like Redux, React-Router, Immutable.js, Axios, Jest, Next.js, Create-React-App, GraphQL, and whatever weird plugin you need for your app.
And, as Jake points out, you either need to go all in or not at all—you can’t really incrementally add Reactness to an existing project.