The huge island garrisons can be tough and highly irritating, but remember that in a war they have to be lucky every time and you only have to be lucky once. All it takes is one battle to go your way and their entire army will be wiped out, unable to retreat. Now I admit an 80K army on Rhodes will be a real challenge to uproot. I haven't personally opposed an island force that large, but I have taken out many in the 30-50K range. Have 2,3, or even 4 waves of assault armies and stagger their attacks so the defenders don't have a chance to recover their morale. It will be costly, but for you as a superpower it should be well within your means. And if the objective is strategically or aesthetically important, it is your duty as a superpower to take it, whatever the cost.