If you want a larger DoF, stop using a macro lens
Short DoF is almost a defining characteristic of shooting macro, and that means if you're manually focusing, you have to be spot on. As you can see, even at f/10, your DoF ends up being shorter than the length of an ant.
As for focusing, just practice. Get a normal lens on the camera, and go around to random things around you and focus on them as fast as you can. I personally find that focusing through something and firing the shutter as I pass through perfect focus is the fastest way to do it, but you get one good shot that way. Taking the time to dial it in and keep it there will let you take advantage of things like motor drive.
Also, don't forget you can move your camera/body to adjust focus as well.