I agree with Sahil. If Unity is a requirement, best way is to use just that to create your app.
If you really want, it should be possible to use Flutter and Unity together. Using Flutter Unity Widget. Although I wouldn't recommend it just yet. It's too early days.
If you do end up using it, I would be very interested in reading about your experiences.
Ru_Co and Sahil are totally right and nothing else to be said. But with your past experience, if you had to choose between RN and Flutter, who should definitely learn Dart and Flutter. (I insist this advice has nothing to do with Unity and your current project...)