Sorry, I just saw this. I’d be tempted to get a less powerful diopter such as the Subsee +5 and keep the SMC so you cover more bases without the cost of changing your housing. The G7X certainly has a better macro mode than the RX100ii but comes with the issue of changing port for wide angle, not a problem for me being used to shooting DSLR but may be an issue for people used to the flexibility of compacts and their interchangeable lenses.
You’ll always find a compact to be a challenge for skittish critters, some things are simply not possible, all setups are about understanding and living within the limitations and making the most of the opportunities the setup offers. This is true of the most expensive setups as you limit yourself to subject matter, or the way you deal with photographing that subject matter, by lens choice.
That said, I personally prefer the interface of the Canon over the Sony, find its usage more intuitive and fractionally more ergonomic, and find the macro mode and the white balance far superior to the Sony. IQ seems much the same.
The upgrade kit from Nauticam is still available to my knowledge but recommended fitting by an authorised dealer as it involves changing a number of key components and specialist tools make this job much easier.