Gotcha. So first and foremost, I would stay the course with Koans. It has been ~3 years now since I did them and I remember them being rough and having to check the solutions 65% of the time, but it was really helpful to see how you'd approach problems in Kotlin instead of Java through Kotlin syntax.
After that, #spring has been a big backer of Kotlin. If you are comfortable with Spring today then take a look at what they are doing with Kotlin. Theoretically, there is nothing stopping you from writing some Kotlin today at your day job because the interop is so good.
As an #android developer, I've been working with #ktor lately on a backend project to help me learn about Google cloud. Ktor has been amazing and it's heavily based on #coroutines so you'd be able to get some experience with how those work.
Honestly, I would focus much more on a personal project or finding a way to get Kotlin where you currently are working. You'll learn way more from building something than going through additional courses or reading books. The kotlin lang documentation is a wealth of knowledge, I often link directly to their articles instead of some medium article because they tend to do a great job explaining things (albeit a little dry at times). Using #detekt sooner rather than later can be nice too, the lint checks are a good way to have code review for general things, oftentimes you'll learn about language features or it will push you to do something differently because it violates a rule.
Hopefully this helps a bit. Also feel free to ask questions in the different slack channels as you run into issues. I've found most channels to be fairly active and there is usually at least one person happy to help out.