Go with the ASP.NET Core. It is a very mature technology now and there are tons of documentation, tutorials and support you can find online. Also ASP.NET Core Web API plays quite well with the React. It is easy to implement the entire back-end in .NET Core (APIs, authentication, database access layer...) and if you need any third party package, I'm pretty sure you will find and implement in a form of a NuGet package. Who knows, maybe one day you'll need to create a mobile app and with a fully functional Web API, it would be more-less easy task to build a mobile app on top of it.