I recommend to get the flash off the camera.
The easiest and most reliable way to trigger it off-camera is using a radio triggers, such as the
Cactus V5. The triggers will enable you to use any flash, including old ones with very high triggering voltages that may harm your camera.
No need to use P-TTL for macro photography, AFAIC. Use manual settings and get reliable and consistent results.
Add a light modifier to your flash (e.g., a portable softbox) and you're golden. For macro photography, the modifier does not have to be huge as the subjects are small.