Looks auspicious.

Me would spare Shimazu as long as he is strong enough to seriously defend himself. If he gets careless you could whipe him out of northern Kyushu in one big offense without taking much losses. Meantime you can let him expand over Shikoku to let him run in whatever dwells there. Still you should take into account that with the velocity you are moving east you will own western Honshu faster than Shimazu takes over Shikoku, so it will probably anyway be you who is dwelling there.

Good luck!