The AI no longer declines deals based on the fact it will allow you to earn too much money, so the old trick of removing ships from trade nodes and then replacing them after the deal is struck should no longer be necessary. Supposedly.
Supposedly remains key there sadly, because I got a trade agreement request from my Fukuhara Taira clan to a Kyushu-based Hata clan to go from Low to High by sailing my trade ships away from the trading posts just yesterday evening, so we can safely disprove that as having been fixed sigh.