Kotlin is a chance to get away from the cruft and annotation magic of traditional Java.
I use Ktor and it's everything you need for a modern API (and can be deployed as a WAR if that's why you want Spring Boot)
Ktor's API that will be familiar to anyone who's used similar "neo-web frameworks" in other languages like Gin or Express, and it allows you to architecture your app in a way that makes sense to you (so if you need DI, you can bring in something like Koin, but you're never compelled to by the framework)