For that many people, I wonder if private clubs are an option.

My HKG friend goes to these private clubs on HK Island for meals and family functions, and he says the dim sum at those places are even better than at acclaimed restaurants. Thus he doesn't even do fine dining at restaurants in HKG. I go to HKG every year, yet I've never been to these clubs and don't know how they work. Perhaps someone can shed light here. But maybe it's an option worth looking into for a party of 20.