Splitting the app into microfrontends is another good option and reduces complexity by quite a lot. I migrated (with the help of a number of teams) in my previous job a huge monolith that contained React, Redux, Angular 1, to a microfontend architecture. The only common state was the user context for which we relied on the context API. It worked beautifully. Simplified so many things and the code was much easier to write, read and maintain.