Well Micronaut supports Groovy, Java, and Kotlin so switching to Java doesn't make it any more or less easy to port to Micronaut. Micronaut just doesn't have the convention over configuration that Grails has, so it's more like Spring Boot, having a lot of annotations. If you don't like the dynamic side you can always use

@CompileStatic

or the Enterprise Groovy Gradle plugin, which makes static compilation the default, but is configurable and has enforcement based on the configuration.