Just eliminate all shooters ability to fire over walls. Or, if enclosed town center cannons are a concern, cut their dps against defense buildings in half. Of course then their scan frequency would have to be increased or they'll spend the entire battle whittling away a few buildings while stray ambushers shoot them.
If they must keep their range because, longbowmen (which become muskets after a few months playtime, at most, making that point moot), then they should lose the 15% dps and loot bonuses. The range more than makes up for it on such a cheap and spammable troop.
With paratroopers and their insane hp to take out and/or distract mortars or decoy spam there really is little to stop them late game.
I'd prefer all other nations got a boost too, but look who we're dealing with here. The same crew who decided it was easier to double sabotage/protect/betrayal's space rather than try to make barrage/first aid/demolition viable. Expecting them to add more punch to seven other nations without breaking the game for months seems a bit much. I've been wrong before though.
Do something for the poor Greeks and Japanese though at the very least. I can't imagine the nightmare of finding scrolls and silk at high ages and medals, but that's a story for another post.