When using flash, I tend to use the native resolution of my camera sensor which is 200 (I think yours is the same). There is no value in going down to ISO 100 since you don't gain an improvement in image quality.
Your flash has a guide number of 138 feet at ISO 200 (I looked online
). This means that it is powerful enough to provide adequate flash power at 138 divided by the f/stop. So at f/16, your flash should be good from up to 138/16 = 8.6 feet away. Even at ISO 100 it is good from 6 feet away. That should be plenty for most herp work!
Make sure your flash is set to Auto or TTL not to some reduced power setting.
Why are you shooting at 1/60th? Why not let the flash choose its own flash sync speed for your chosen aperture?
Just switch to aperture priority, set the aperture you want and let the flash choose its shutter speed to sync. You camera will sync at speeds of up to 1/500th of a second (or faster with some flashes).
Having a shutter speed longer than the flash is lit doesn't help anything since the subject is correctly exposed during the flash duration. If you leave the shutter open longer it can actually allow blurring or ghosting if there is enough ambient light. The flash duration for your flash is only 1/900th of a second at full power so you really should let the camera decide how long it needs the shutter open. Cameras are pretty smart now, if you allow them to be.