I myself don’t really know many good tutorials although I’m sure there are a few out there. https://kotlinlang.org/docs/tutorials/ is a good point to start.
After going through koans you can just convert some java files to kotlin (Intellij feature) and see how it feels. Switching from java to kotlin is quite simple. You should take a closer look at how higher order functions and inlineing works in kotlin but appart from that everything else should be pretty much intuitive. Another pain point could be immutable vs readonly vs mutable. Lot’s of people seem to think that
List<T> are immutable even though they are readonly.
The kotlin eco system (at least on the jvm) overlaps quite a lot with java. This is one of the big selling points for kotlin. Using java libraries is simple and just works as if they were written in kotlin. So yes and no. You can rely on java libraries, but you can also find kotlin alternaties. One big one would be kotlin coroutines.
In the end that depends on you and your project. Not every language is the best for every situation, but I’d say that kotlin is supperior to java in most if not all situations.