The problem with loading all your troops on a ship (which I'd only recommend after taking Sado as Echigo is almost as bad a stack magnet as Owari but I've almost never seen anyone land an army on Sado) and sending it to Hizen is that then you might as well be playing the Shimazu.....

Having said this having finally conquered the Takeda, Hojo, Satomi and Ikko Ikki provinces I found myself with very strong allies blocking my expansion to the east and west so I did end up sending all my monks and ninja on boats to Kyushu to raise some revolts from the Christian Shoni followed by an army to take over the new rebel provs - but this process took so long that I only hit Realm Divide in the 1580s and gave up around 1595 as there was clearly no way I could conquer the additional 18 provinces I needed for domination by 1600 without a rather graceless rush.