Artsy.net has had quite a bit of success with a brown field RN app. We adopted RN in our iOS app in 2016 and gradually used it to render more and more screens in the app. In 2020 we decided it would be nice to have an android app, and we spent the last year incrementally refactoring the app infrastructure from Objective-C to TypeScript to support that. Crucially we did the entire thing with a tiny team of 1-3 people and without blocking any product engineers from shipping new features.

We had occasional tooling issues like you describe but all fixable. I'd be happy to go through the same process again.