Is there a visibility modifier between public and internal? This is for a library project with many gradle modules. API usage is meant only for all modules and not external consumers.

nope

Okay, is there something similar to Android's @hide for public APIs?

package-private wouldn't help in this case either. All a consumer has to do is put their class into the same package, right? And then they have access that is not desired. I'm not familiar with @hide, but doing some SO research, it appears it merely hides the method(s) from the documentation. Can't get to Dokka docs right now. Does it have something to hide a function from the docs?

Android sdk library build process removes all methods/class marked as @hide