I go authority all the way with Japan, more kills equal more GG equal more GP. Goddess of protection is my go to, you get a lot of yields with your barracks and walls, and the extra 10 hp is strong for early warring.
Samurai are a very good unit if they are well tended to. They score xp quickly, which translates well as they upgrade to tercios. So they are both a good unit for their time, and leave a solid legacy later on.
Late game it’s all about autocracies lebenstraum. You should have a bucket load of extra GGs at this point, enjoying cutting through your opponents territory like a lawnmower but just placing citadels down