This is a gorgeous cake! I personally like to use thinned bc for my scrollwork. The same consistency that you would pipe words with and I would probably use a #3 tip on this one. JMO
A thin or med consistency b'cream should work just fine.
Rather than thin w/liquid, use piping gel OR Karo syrup. It will help give some stretch to the icing that will give you a smoother piped decoration.
I just tried the Karo Syrup technique: I'd never heard of it before. IT'S GENIUS!
Thanks so much!