> My Scala started off much prettier than my Groovy code usually is, but by the time I finished it was an abomination

> Sometimes I think I should just use Kotlin, but then I would miss the ability to drop seamlessly into dynamic mode

Have you considered Clojure for this use case? That might alleviate your problems with using Kotlin and Scala, without having to resort to a single-target language like Apache Groovy. After all, unlike what Clojure, Kotlin, and Scala all offer now, we're never going to see an edition of Groovy that targets Javascript, Android, or native CPU.