While I am not a big fan of kotlin but I guess for your use-case it would be better, since you need only an android app and learning es6 , react & react native will definitely be a time consuming task since it has a steep learning curve and has a lot to learn, but it is more flexible and useful than kotlin though.
I would agree with that assessment.
That is, if you do want to stick to Android-only.
If you might branch out to having an iOS version as well, using a cross-platform approach could save you time.
React Native is one way to go, but it's not a small barrier to entry, I would say.
An easier solution could be Kotlin Multi-platform, once you have already done a Kotlin-based Android app.
The language doesn't matter for your choice of backend, i.e. AWS and any other backend services can be called easily from all of them, and most services have SDKs and libraries in all major languages you could use.