Instead of creating references in global singleton and creating same functionality for every instance you need to reference use this resource.
Instances that fulfill same functionality, for an example a level camera, should add(self) and listeners that are depending on this reference can listen(self, OnUpdateCallable) or use updated signal, or just check .node.
add(self) will remove reference automatically when instance exits the tree, and same for listeneres using listen(self, OnUpdateCallable) will be disconnected on exiting tree automatically.