I like Rust as much as the next guy, but Kotlin is the most ergonomic programming language I know. So my approach is to use Kotlin by default and should it some day become clear that the service is a bottleneck (or if the cloud cost can be lowered substancially), to only then rewrite it in Rust. At that point the service has probably already gained most of the functionality it'll ever have, which should make the Rust conversion as straight-forward as it can be.
Have you ever coded groovy? If so, besides that it is nigh a dead pang, what does kotlin offer?
Kotlin is more type oriented, while offering a lot of niceness in terms of syntax (like not having to have a class that surrounds main).