My personal thoughts, which are worth what you paid for them, is that around .300" is "ideal" thickness in a full length sword. There may be very very little cross section of that sword that remains a full ~.3" when complete, but I think it's optimal because you keep that thick cross section where it's needed the most around the handle junction/tang shoulder(whether for strength or balance reasoning), and it gives you a lot of options when it comes to tapering and profile shape.
But I also don't think .188" necessarily sub optimal. It simply constrains how much taper and freedom to profile you have.
That opinion may be completely worthless because I don't know anything about swords outside what I've read and my colloquial understanding, looking at it from the perspective of a machinist, I'm almost always going to want more stock than necessary because I can always take more off.
Ken, for your bevel grind I definitely think you should do a flat grind and slight convex. I only hollow ground my Waki because I happened to be making it when I got my first 12" wheel and thought "why not try it out."
