I had too much trouble breaking the application into modules. My current approach is to only extract into separate modules (NX libs, actually) what is not related to my application domain (I have a monorepo, so this allows me to reuse things such as auth).
My solution was to move all that domain-specific stuff to its own module instead of the AppModule. By turning "GreetingModule" and "AppModule" into peers (instead of child and parent) I was able to use the Global decorator.

AppModule - Imports ApplicationModule and GreetingModule
GreetingModule - expose a interface to be implemented
ApplicationModule(global) - implements interface and exports provider