Yamakawa on rakuten doesn't have any in stock, but they do have 240s in both white and stainless (aeb-l i think?). They've also got a 300 in white.
Naniwa Superstones in the lower grits are quite soft and wear very quickly. They cut just fine, but you have to flatten them constantly.
My preferred progression is Chosera 800, Chosera 2k, Naniwa Snow-white 8k. Rarely use the Chosera 5k anymore. The Naniwa Professional is the same as the
Thanks for the picture! That's what I had in mind and am going to do for this knife. Should be fun!