It might be better to have any influence above 60 drop to 60 upon CS surrender and reset all issued quests for any player. It turns out that none of CS's defenders could come to the rescue, but CS retains the memory of past good relations. For a former ally, you can increase the rate at which influence decreases by 1.5 times for 20 turns.
Also, when CS capitulates, you can reduce the chances of destroying buildings or destroy only walls, castles and the like, so that CS in the late game can start building up a new army for his defense, instead of building forges and circuses. 10 units will most likely be produced in at least 20 turns. Plus CS needs to produce workers, repair damage and build new buildings.