I think it would be fun if Berserkers kind of worked like the Viking unit in Starcraft 2 that can be both an air and land unit (stronger air unit than land unit). When Berserkers are embarked they turn into a Longship and when you move a Longship onto land it becomes a Berserker once Berserkers have been researched. That way you can prebuild the Longships and then do the timing attack when you hit the tech for Berserkers. To make it a little more balanced you could require that the Longships be upgraded with Gold or Faith before it becomes Berserker enabled. Maybe the name of the Longship can then change from Snekkja to Skeid to reflect this.