Thanks so much for posting on our forum. I appreciate it.
Now to you..
When I read detailed questions like this a knife or two often jumps into my brain. You are looking for a hard steel, santoku that will take and hold an edge for a long time. Seems like you're open to western or wa handled knives but you need it to have a 50/50 edge and octagonal handle. Looks like you are open to carbon or stainless. Did I get it right?
Here are a few I would recommend to you.
If I could recommend 1 knife for your needs this is the knife. It's stainless clad with Aogami Super steel on the edge. It's lighter than all the knives you have (we do have lighter ones) and it's a great performing knife.
This knife has everything you want. It's thin, light, nimble yet takes and holds it's edge. Shosui uses the same steel as Kanehiro and it's good stuff. It also has a 50/50 edge grind.
These are excellent knives as well. Same steel as above. Same octagonal handles so they will work for you too. I love Moritakas and we sell tons of them for good reason.
I hope that helps. If you want some fully stainless suggestions let me know but I think you would have fun with one of these.