If you are the sole developer working on migrating the project and you want to have the frontend in React.js why not use Node.js/TypeScript instead of Scala for the backend? This would save you from context switching between two languages and consequently increase the development speed.

Also why the rewrite? If it works (in PHP) it works. Perhaps instead you could maybe add some frontend JS for better caption UI/UX?

Does everything have to be done in React nowadays, as opposed to plain static HTML? (Speaking as a React/Node dev myself)