My experience, mostly on larger birds, is that I get decent bif images at 1/3200 sec. at up to iso 6400 using f/7.1 on the 70-200 lens, vr off and D500 body. So, I think you might get a sharper image if you are willing to accept a slightly higher iso. It takes some experimentation to find out what works best with your lens and body combo, but generally speaking I find shutter speeds of 1/2500 sec. and faster work best for bif.
I love the plummage on your kingfisher image, truly a beautiful bird. Thanks for posting!