As I googled the internet for alternative recipes, I realised some people used roasted duck to make kiam chye ark soup (ie salted mustard cabbage duck soup). Some even used the bones/ leftovers from roasted duck. Such a good idea!
I'd definitely do that next time.
In the meantime - I think the current recipe I have is good enough and simmering it for 2.5 hours in a cast iron pot helped made the duck meat so soft, it was literally falling off the bones. Adding a splash of brandy in the last 5 minutes of boiling would really add a kick to this soup too! Since I didn't have any at home, I added some Chinese wine.
Before the 2.5 hours of boiling..
And after... yummy!