Believe me, one of my most powerful abilities has a long animation and is off-gcd I've acquired QUITE a lot of experience with that (it's Jump)
I know well that you can use your off-gcd abilities before the GCD finishes ticking although you will only have time for one. Fast Blade has a hilariously long animation, actually, not because of your swing time but because of the effect on the target, the animation cancel point on Fast Blade is not after the swing is done and your arm is outstretched but after it's closer to the body in a resting stance.