The "flattest" are certainly the Four Seasons, Sydney Harbour Marriott (there are a few Marriotts in Sydney) and the Pullman Quay Grand (pretty much directly opposite where the ships dock across the quay, very close to the Opera house)
I still don't understand this Australian hospitality businesses WAITER'S) refusal of split bills.
Any pub waiter in Austria, Germany, Switzerland etc will always ask if you want to pay together or separate. AND they likely remember what everyone ordered, AND do the sums in their heads (or, at worst) with pen and paper. Well, at least that's what I was trained to do, way back when....
For something on the simple side, City Extra at Circular Quay may be OK. Their upstairs dining area is hardly every busy. 'American Diner' style of food in generous portions.
For something nicer, look at Morrison's Oyster Bar and Gril in George Street. Less than 5 minutes from Circular Quay. Upmarketish pub fare. Several areas and private rooms. FRIDAY afternoon and evening may be BUSY.