With my macro lens I can go so close that even ring flash doesn't really work, how can you light up your subject especially if it's small?
If you're so close your ring flash doesn't bear on your subject, you're pretty well limited to side-lighting the subject. One option might be a diffusion ring attached to the ring flash. A ring of LEDs, might also do the job, if central light on a near-flat surface facing the lens isn't critical. Depending on what kind of catch light is acceptable, you might prefer to put your subject into a diffuser box, either constructed from flat light panels or lit as evenly as possible from out of field.
Another option to consider is using a smaller ring flash, or one that has an inward angle -- what's available here will likely depend on the lens you're using, and its filter ring size (or rear element diameter, if you're using a reversing setup of some kind), but I've seen (pictures of) LED rings that would fit around the rear elements of some lenses.
One solution is a lens with a built-in light. The light doesn't need to be very powerful since it is very close to the subject.
Closeup/macro buffs tend not to use ring flashes (which cause flat lighting) as much as elaborate hand-made snoot constructs. My personal insect hunter gets along with "old stock" components, namely a suitable flash reflector and a number of (achromat) closeup dioptres. You still may want to have some white cardboard to hold behind your subjects lest the background may become too dark:
However, I recommend that you do a web search for people who may be considered macro giants: those really usually have quite more elaborate self-made light guides that will typically provide reasonable shading as well as background illumination, and those will usually provide better visual depth impression than a ring flash would.
The most relevant advantage of a ring flash is that it will rarely fail to get the light where the view is. For dental photography (or insects crawling into narrow blossoms) that can be important. For the focusing distances you indicated, there is not much of a chance for a hollow to interfere with the view, though.