You're right about the Japanese knives, I'll qualify my statement: the edge on a really good sharp western
kitchen knife is about 40 degrees
Sashimi knives are terrifying, and very high maintenance.
My experience so far with Japanese leatherworking knives having an iron back and a thin but very hard steel edge are that with the extremely shallow grind angles they chip if you look at them funny, but they are a lot more robust with a 30 degree (30 + 0 chisel grind) edge and still cut incredibly well.
I'm sure that's user error, and I need to find a Japanese expert to teach me how they use them and figure out what on earth they use as a cutting surface, because nothing I've tried stops the corners chipping.
Anyway, within reason you can grind the primary bevel as shallow as you like. In some ways a 20 degree primary bevel is better than a 30 degree one because your actual edge angle always has to be greater than the primary bevel angle.
That means if you go with a 20 degree primary bevel you've got the option of a 30 degree secondary bevel if you find a 21 degree edge chips or blunts too easily. It still won't be as robust as a 30 degree primary bevel though.
So, assuming a normal western type blade made of a chunk of tool or spring steel and hardened to around 58 rockwell, I'd still recommend a 30 degree primary bevel