So the 8 direction capx doesn't work right for isometric. I can see that, which isn't a problem since it wasn't made with isometric in mind.
So the problem becomes how to find the closest angle as in your image. There probably can be an elegant formula to do it nicely, but barring deriving that you can check against all those angles to see if it's close.
But wouldn't it be simpler to do all the motions from a top view and set all the visible stuff to an isometric projection? Then the 8 direction capx is perfectly applicable. I doesn't matter that 45 is an animation drawn at 22.5 degrees, because when the angle of motion is changed from to view to iso it will be 22.5.