I whined incessently on this forum about fillet knife being hard to sharpen, hours of zining away on them, and going dull at record speed. I fought that for years. I won't go into all the crap I discovered about most of them, but the only one that worked even reasonably was is a DR. The steel is kind of soft but won't rust and sharpens quickly.
Phil Wilson saw my posts and offered to build me one in S90V. Problem solved. The first thing is, it cuts and cuts and cuts without going dull. The second thing is, using a course and fine, 8" Spyderco ceramic sharpener, I can restore the edge in about 2 minutes. The flex thing does not seem to be an issue. The knife cost a bundle, but so did my years of experimenting. I wound up with a drawer full of crappy knife, and some of them weren't cheap either. My recommendation is, upgrade the steel and design and life gets a lot simpler.