Kotlin is a good way to make a lot of Java frameworks a lot nicer to use. Extension functions can smooth over a lot of boiler plate. Spring Boot is a good example of that. Using that from Java is a form of masochism. Using it from Kotlin is just a lot nicer and easier. Spring has had excellent support for Kotlin out of the box for many years and even before that it was already quite nice. I made the switch about six years ago and haven't looked back since. I have been using Java/jvm since 1995 so I've been there, done that, and seen it all. Not touching any Java a lot these days.

Gradle is not ideal but also not that hard once you figure it out. I tend to use template projects on Github so I don't have to reinvent that wheel every time. And with most build systems once you find yourself extending them, you might want to consider if a simple bash script wouldn't be the better solution. I've seen people jump through a lot of hoops to get docker containers running before they run their tests. That's a 1 line command in a shell. So my github action does : 1) docker compose up 2) gradle build 3) docker compose down. Doing the docker compose business from gradle has zero value for me. But I've done that as well.