I never worked with Niagara and after some time looking at it, I didn’t find how to create similar code and test the solution. But from experience in creating SceneComponents I can suggest you try USTRUCT(BlueprintType, meta = (BlueprintSpawnableComponent, IsBlueprintBase="true")).
Sry, if that didn’t help…
BlueprintSpawnableComponent allows the instantiating of classes/structs in blueprints.
IsBlueprintBase="true" (for structs) and Blueprintable (for classes, and isn’t a meta but the same parameter as BlueprintType) for creating derived objects by blueprints.