My team uses Kotlin + Dropwizard + Postgres a lot. I would absolutely recommend trying Kotlin out on your next API. You will find the code very easy to learn coming from Java, and it is honestly a joy to work with compared to Java. You will find your code much more concise yet more readable. Also, IntelliJ gives you all kinds of hints for how you can make use of Kotlin language features.

You can also easily handle nulls in Kotlin, since values have to be explicitly nullable in order to be able to have one. This is great for your application model layers. The val/var paradigm is nice so you can stop having to remember to write 'final'.

If you ever get stuck and don't know how to write something in Kotlin, you can just write it in Java and either use it, or just paste the code in IntelliJ and it will convert it to Kotlin.

Lastly, Kotest + Mockk is awesome--super readable testing.