I only know very limited things about development in Android, but the industry definitely seems to be shifting towards the React Native side to build apps for mobile (both iOS and Android). You might want to consider that as well.

In fact, I went to a beginner React Native meet-up on how to build apps for mobile, last weekend and it made a lot of sense to me to go the react native way.

The event was recorded and the video is available in this link: hashnode.com/post/summary-of-3rd-react-native-mee…

Anything is better than Java 6 for Android dev. Java 8 support is now available which is a huge improvement, but Kotlin is still much more fun to work with. Flutter is also getting more usable, but not ready for production apps yet. So your options are Flutter from Dart, Kotlin, Cordova (which you can also mix with Dart if you don't enjoy JS), Ionic (I didn't enjoy the TypeScript experience) and React Native ( which I haven't tried yet). If you're from a C# background, Xamarin is also an option

I'm not a Mobile App Developers, but the recent Google I/O presentations and hands on, on Kotlin looked very promising over Java (in terms of how less a code is written compared to Java). But I don't really know. One of them is this video: