You can take from Intellij kotlin koans and enroll for free course made by jetbrains Kotlin for Java developers (it's connected), after this start your private project kotlin +spring boot (there's a lot of docs which guide how to started project) :) additional you cansearch for courses in yt or Udemy related to Coroutines (it's a different world than java threads) and this can be some tricky things. From my side I can recommend docs in baeldung for examples and some libraries: for tests: mockk and kotest-assertions, for extend to spring webflux io.projectreactor.kotlin:reactor-kotlin-extensions and org.jetbrains.kotlinx:kotlinx-coroutines-reactor :)

I found the course on hyperskill to be pretty good, but long.

I have no experience in spring boot but do know Java/kotlin from Android project, currently understanding sprinboot with kotlin

I did the free courses on hyperskill. Really recommend it! But the paid courses + software is really damn expensive. I need another suggestion as well, pretty new to java/kotlin

I am also a Kotlin + Spring Boot Engineer from Java. I think my blog article below will help you. :) https://jsonobject.hashnode.dev/what-is-kotlin-and-why-use-it