I am puzzled that this camera does not do well at ISO800. My Sony RX100 is not great but still acceptable
For what concerns depth of field it will be an issue with the dome port but with the flat port is totally unneeded to push small apertures. I would not put magic filters on the lens is too restrictive.
I would think push on filters are the way to go with the Inon UWL-H100 this lens works fine at f/3.5 pin sharp on my GX7 no need to go smaller apertures as long as you don't put a dome.
I am looking forward to testing my GX7 with the Inon UWL-H100 results are excellent with the flat port and you can do full zoom without soft corners
The camera performs very well and as I can use relatively wide aperture such f/4 I can shoot low ISO and decent shutter speeds
I think this bridges the gap with other cameras where you need to push two stops on the aperture and end up with higher ISO