Second for Hinoura. Also Mazaki if you could find one; I know he’s made at least one 240mm sujihiki but I can’t find any for sale now. I bet that Kochi is good too. The Takayuki Blue 2 is indeed a stiff 240 but note that it’s very asymmetrically ground and beveled so I see it as more purpose specific for protein. Surprised to hear a Watanabe felt flimsy, but I also get that feeling from my Kurosaki Raijin. Tsukiji Masamoto maybe?
Stiffness in a sujihiki is a function of thickness at the spine and edge, height, and taper, and to a lesser extent steel; Mazaki, for instance, is thin over most of its length but slightly taller and very fat at the handle and heel, so it’s on the stiffer side. Tou is quite narrow (height) but thick at the spine, so it’s also stiff. That Takayuki isn’t particularly tall nor thick at the spine but it’s thicker over its height because the grind is flat on one side and convex on the other and sharpened accordingly, so thicker towards the edge, and also the solid blue steel probably has something to do with it. Shiro Kamo isn’t quite as tapered as Mazaki and Hinoura but is even taller so it gets its stiffness there.
I too am looking for a 255mm sujihiki, some nominal 270s made in Sakai should measure in about that range. Kagekiyo and Hitohira Kikuchiyo are the ones I’m looking at; also someone on here showed one by Yoshikazu Tanaka that’s about 260. I think the only production ones I remember specified at 255mm were Masamoto’s budget lines which would almost certainly be too flexible.