A few more things to share...

On merging units:

What I did with pairs of damaged units sometimes is merge them, resulting in one unit at full strength, and one much weaker unit. I split the stack, moving the full units forward to attack or threaten my enemies, while leaving the weakened units to defend a fort or castle. The weak units still count as full for purposes of repression. Also, by merging very damaged units, you get a better looking stack (less ragged banner), and reduced upkeep costs.

On trade:

It seems to me that the number of Trading Ports is a "soft" cap on the number of overseas trade routes, including foreign ports, and if you are over that "soft" cap, you can't gain any further clans as trade partners, although you can still get more foreign ports. I have 5 trading ports: Tosa, Iyo, Buzen, Hizen," and now Satsuma, and can't get a fifth trade partner beyond Soga and Miyoshi by land, and Ashina and now Hojo by sea. But I do have two foreign ports. Even gaining the Nanban Trading Post in Bungo hasn't helped. I burnt the Nanban Trading Post to the ground, for the purity of Japan.

Smeelbo