In short, if you are using Spring and are ignoring Kotlin, you are missing out a lot of good stuff. Or in other words, the same that happened to Java on Android is now happening to backend Java. You can still use it but why would you when things can be so much better with a simple upgrade to Kotlin (I've migrated tens of thousands of lines of code, it's easy). And yes Java 9-14 added a bunch of things that are definitely nice/useful but it doesn't come close to Kotlin in terms of expressiveness and language ergonomics.
Also, KotlinConf was on last week. There was a nice presentation about the state of kotlin-js, which is getting some nice updates in the next year including much better dead code elimination resulting in very compact minified code (< 100kb is now feasible for simple stuff), better integration of existing typescript type script defifinitions (i.e. support for most relevant npms out there), better build tooling in the form of an improved gradle plugin. Or put different, Kotlin might start doing to typescript/javascript what it already did to Java. IMHO it's a more capable language than typescript, which I consider to be pretty nice but it just lacks a bunch of things and comes with a some ugliness that relates to backwards compatibility with js.
In related news, Kotlin native is also happening. So, Kotlin is joining a list of full stack languages that used to be really short.