You might look closer at react-native-web. It's early days and React Native can be painful at times however it does offer the idea of using one codebase for bot

You might look closer at react-native-web. It's early days and React Native can be painful at times however it does offer the idea of using one codebase for both mobile and web. It does require some care, flexibility and thought in design and development. For the API, look to PostGraphile for GraphQL on top of Postgres (and of course, if multi tenant, there is row level security which is the direction I'd go today).