I want to throw Ionic and Capacitor (or any other webview based toolkit with your favourite SPA framework) into this mix, I believe for a lot of apps its probably a better option than either Flutter or React Native. I also don't and believe in "one true cross platform toolkit", each has its place. So:

Hiring - The OP suggests React Native has a larger pool of developers than Flutter, very true. But Ionic/Capacitor/etc you have access to an even larger pool of developers, any proficient font end web developer.

Sharing Code, Knowledge, and Developers - Again I believe Ionic/Capacitor/etc are even better than React Native here, you can potentially reuse even more of your code/views/logic. In fact your SPA web app version is your mobile app, except you then have access to native apis with Capacitor.

Developer Experience - Dev experience with Ionic/Capacitor/etc is as good a React Native at this point. It has all the hot reload features we now expect from any SPA framework. You can even now you Vite for super speedy development.

Performance - A webview based app on hardware less than five years old is on par with any React native app. The performance of web views is not the issue, it's the underling app code, modern SPA frameworks and good engineering is all you need.

Unified UI Experience - Firstly customers don't really care about a unified UI, they have been trained after 20 years of using web apps not to care. If it works it works. Having said that Ionic have done an incredible job of replicating the Material/iOS components.

Native Integrations - Capacitor has all your usual apis covered, if you need more it's as simple as with React Native to drop down to native with Swift or Kotlin. I'm a fan of using NativeScript with Capacitor, it gives you access to all native apis straight from your JS codebase.

Internationalization - Standard SPA internationalization support, easy.

Built-In Navigation - Use whatever your SPA framework uses.

Web Support - Capacitor is the clear winner here as you have built a SPA.

Third-Party Libraries - You can use any web/javascript toolkit, along with any platform specific libraries for iOS or Android that you need.

Obviously its not the right platform for all use cases, but if you are just starting out small with a tiny team you only have to build your app once. 90% of your app is probably the front end, your don't want to be build that twice until you have the traction and teams to do it.