I’ve looked at Kotlin but haven’t used it. It seems like it was designed carefully, and conceptually, I think Jetpack Compose is pretty cool. Is there a (non-Spring-like) Kotlin server-side framework you would recommend taking a look at?
The main Kotlin framework is Ktor.
https://ktor.io/ for Kotlin server side. It's not as large/complete as Spring Boot but you may find it preferable.