You should try to get montalbans hideout and a couple of lost cities with one crew. After a certain period of time the crew morale drops no further, so it is only dependent on how much money you have (gold per crew member).
I now hav 500 000 gold and my crew stays happy. The last time I divided the plunder was 150 month ago. I'm now aiming for a perfect game, I just need the last spanish title
Are you all sailing with a fleet? I only sail with my large frigate and just keep all the ships i capture to store the stuff. In harbour I sell all the ships.