One Special Dinner for group of 6 aged mid 40's to late 50's
Where would you go? Would be happy with French,, Mediterranean, Italian, Fish or Steak. Should not be too loud or cramped. Celebrating milestone birthday.
My first choice given what you've described (staying on Brickell, not too cramped or loud, special occasion, first mentioned French food) would be the new DB Bistro Moderne downtown in the JW Marriott Marquis.
I've only been once, so can't give a full run-down, but it's a very "serene" dining room (despite calling itself a "bistro" the room has a more formal feel), and the execution of the food is very precise and meticulous.