This is how I would do it for on camera flash, If you know that ambient is ISO 1600 f1.8 1/500 sec, based on your EXIF info of your first shot, The first thing I would want to do is lower the ISO, if i go down to ISO 800 that is one stop, I would actually go to ISO 400 2 stops lower than what you need to shoot ambient, I like shallow DOF so I would leave it at f1.8, Shutter speed I would then adjust one stop to 1/1000th's. Set camera to manual with these settings and let the auto flash do its work. You can meter the flash to get there as well, this is what you would need to do if you used strobes or flash setups off camera. You are setting your camera to 3 stops below the ambient setting and letting the flash fill in the rest of the light. If you still find this to harsh you can adjust one or two stops down to your liking.
Hope this helps in a quick explanation.
Pete