I recommend you to go with Android(kotlin) development and Swift for iOS.
Here are the pros:
- As a profesional app developer you want to know the native patterns better, so you can take out the most from the platform.
- You won't rely on third party components as much.
- You'll have immediate access to the latests apis as they are released in the native framework.
- Faster testing, better debugging experience.
- Easier integration with CI tools or automated test tools.
- Shorter build times.
- Writing in two languages has also it's advantages, you can learn things from Swift 4 that you might love.
Cons:
- You'll have to write two applications.
There are things that will be fundamentally different in both platforms and you won't be able to build once and release in both platforms anyway. (Android instant App is the first that comes to my mind)
So in my opinion you should start with native development first, later you can change your mind, and go to Flutter but you'll be able to leverage your experience to build your own plugins, and create really advance stuff.
Otherwise you'll be always limited. IMHO.