Crazy viking idea (probably not suited for this mod):
UU = Longship, 3-move trireme
UB = Trading Post, Lighthouse with +1 trade routes
Vikings were renowned as traders too, not just as raiders. The longship is a far more suitable candidate for a UU than the berserker IMO. This way the movement bonus is limited to a single ship type, rather than giving you a bonus throughout the game. Or maybe give Trading Post a 50% trade bonus if +1 trade route is too strong on an already useful building. 50% is not much, but I think it'd be enough to give you 3c trade routes with your island cities.
UU = Longship, 3-move trireme
UB = Trading Post, Lighthouse with +1 trade routes
Vikings were renowned as traders too, not just as raiders. The longship is a far more suitable candidate for a UU than the berserker IMO. This way the movement bonus is limited to a single ship type, rather than giving you a bonus throughout the game. Or maybe give Trading Post a 50% trade bonus if +1 trade route is too strong on an already useful building. 50% is not much, but I think it'd be enough to give you 3c trade routes with your island cities.