I went a completely different path Maltz. I conquered the entirity of Shikoku very quickly, then switched to Christianity. Trying to control the seas on Legendary with conventional navies is just painful, so European ships are a must have IMO.

Making use of the Chosokabe farm bonus and town wealth/research boost from Christian buildings I kept my economy afloat that way rather than worrying about having active trade routes. You can still get a decent amount of tarrif income regardless, and post RD when money is tight you won't hav many trade partners left anyhow.

My toughest time actually came when I invaded the mainland and lost 3 stacks (plus generals) due to my ineptness at commanding large amounts of bow on Legendary. I adapted my tactics and eventually arrived at a successful technique but boy it was a bit grim for a while :) Still was one of my most enjoyable campaigns as a result.

I will be interested if your peaceful approach works out for you!