Yes the online system has you reserved but once on ship, you'll find it pointless. Booking dining reservations online hasn't mattered on my last 2 sailings, one was as recent as last month. The e-class ships have open dining across all 4 Main Dining Rooms.
The Maitre"d onboard will help the OP with a table of that size but might have to stick with the same MDR each night.
Patty