My recent experience of sitting 12 in the traditional dinning room has been similar to yours. We had four people that needed to sit at the ends, it was difficult to talk and we had terrible service. We were always the last table to be paid attention to and frequently the last table to get out of the dinning room.
On a past cruise on the Spirit we had no problem getting to eat at the same time - 16 that time but we did sit back to back. For all of us that worked out just fine and in some ways it was easier to chat back and forth - table to table.