So far I've only found bits and pieces on how to make GraphQL services in Kotlin, but nothing that seems to have wide support or documentation.

Any Java GraphQL library will work with Kotlin

Yes. I'm aware of that. I guess the question should have been more like "What level of success have people had creating backend services with Kotlin and GraphQL. What should be taken to mind, before I start?"

We just made a GraphQL backend / API where I work and it was not that hard. There was one lib with special extensions for Kotlin I think, don't know if we used it.