Lounge looks pretty cool

Tyson is nice - esp. if you are already using TS/JS.

https://github.com/jetpack-io/tyson

I don't use it right now, but two years ago I helped a team incrementally adopt Kotlin in a ten year old java/spring/mybatis codebase. We didn't have any android experience and in the initial few months mostly used kotlin as a better java, avoiding features that would prevent us from switching back to java if needed.

But it worked pretty well - we didn't face much resistence from people experienced with java because they could still continue to benefit from their jvm familiarity, and the language was approachable to new folks who joined us. It also helped that we could just copy paste java code into a .kt file and intellij would convert it to kotlin.

We didn't venture into kotlin's js/native targets but for jvm it worked out great for us.