At the risk of being obvious, the thread's title is "Royal
Belgian Stout".
Just sayin'.
I think your grist is pretty good. I'd definitely opt for Chocolate for a broader spectrum of roasted flavors.
Also agree on the bittering schedule. I wouldn't bother with flavor additions. Don't get hops flavor in the way of the roasted grains and the yeast.
I absolutely ADORE Ardennes, and think it's an excellent yeast for this beer. Unfortunately, I don't think WLP550 is anything like Ardennes, but that might be my palate at work.
Good luck!
Bob