Whats your choice of "micro"framework would be for simple app? http4k/ktor/vertx/micronaut all seems solid choices with kotlin support, I'm confused

If all of them satisfy all of your requirements, roll a die 🙂 Or choose the one which in your opinion leads to more aesthetically pleasing code. Or whichever is better represented in terms of community/documentation, etc. More often then not, there is no the best solution. In such cases choose the one you feel better about if any, or literally number them and roll a die 🙂