Ok a few thoughts:
1) 1/15-1/20 is really very slow to handhold any lens, especailly a macro lens. As a result you'll likely have introduced a level of softness to the photo from that alone unless you were hand holding; with flash dominated lighting you can get away with slower speeds, but ideally for a sharp handheld shot you want a good fast shutter speed - doubly so if the subject you're shooting is living and thus likely to move or be moved by the wind (even if only minor motions).
2) You've got a good aperture for depth of field to cover your selected target; do remember that there is no aperture rule for macro photography, f2.8 as as valid as f13 and any other aperture. So play around and see how it affects the shot and then make a choice for the scene based on what those experiences teach you (if you want hard facts look up a "depth of field calculator" and you can get hard numbers to work with too).
3)
a) Lighting is key and for macro where you're adding light because of the need for a fast shutter speed coupled with a small aperture (big f number) adding light becomes very important - remembering that even on a tripod any motion of the subject can be a problem at slower speeds.
b) Ideally for macro you want to diffuse the lighting and the simplest way to diffuse lighting is increase the size of the light source relative to the subject. This is often done with things like soft boxes, umbrellas or bouncing the flash light off a large surface area (often you'll see this done at parties where the flash is pointing up and behind the person and is then bouncing the light off the walls). Ring flashes are a problem though as they don't give you much room to add size to the light element; a flash bracket coupled with any flash and a diffuser is a much more powerful tool (don't get me wrong ring flash units on the lens can be very good, especially if you want a very clinical light with no shadows).
c) LED ring lights are generally not powerful enough to really give you much of a light boost; as you've found in your above example where you're forced to raise the ISO and slow the shutter speed. LED ringlights can be great for lighting the subject and getting a lock on focus; but for lighting itself for exposure they just don't have the power for most macro situations.
d) A speedlight design flash light not only gives you the power, but also the control since when combined with a flash bracket it allows you to direct, expand and control the flash light. A small softbox like a Lumiquest original and you've a compact setup that diffuses the light and gives you the light you want where you need it- whilst letting you have a good shutter speed of say 1/250sec or 1/200sec (which are about as fast as you can go with flashes and shutter speed on most current market cameras) - combine that with say a nice low ISO and a small aperture and away you go.
