I agree with all Chris's observations.
Compromises are part and parcel of all photography, whether it be size and portability, flexibility, usability, durability, cost and of course IQ. Photography in the aqueous environment imposes an additional major layer of compromises, not the least of which being the need for refraction correction for wider angles of view. Understanding the compromises we are all making is important as apart from helping with equipment choice, it will determine subject selection and approach and best camera settings. Although there is currently a resurgence in interest in more sophisticated water contact optics, the simple dome port with all its optical compromises remains the most practical and cost-effective option for most of us at the moment. Just how much IQ compromise we are willing to accept, particularly at the edges and corners, ends up being a personal decision.
I am sure your 10-18 will perform adequately behind the Fantasea 155mm dome, but assuming that the different diameters are more or less proportionate to their radius of curvatures, I would expect better performance behind the Zen 170mm dome with which I have experience. To a degree it is possible to compensate for the poorer edge performance of smaller domes by using smaller apertures (perhaps no larger than f11 to 16).
Although I am primarily a DSLR shooter, together with my daughter we also use an A6000 (land and underwater) and A6500 (currently only land). I agree that both of these cameras are very technologically advanced and capable at their relative price points. Limited lens choice was long an Achilles heal for Sony E/FE mount cameras. Although this has been steadily improving for the full frame lenses, Sony don't seem to be investing in new lenses for their cropped sensor cameras. This forces Sony cropped sensor users to consider using the FE lenses, but their size, weight and cost negate one of the advantages of cropped sensor CSCs. For underwater macro use we use the Zeiss Touit 50mm f/2.8M Macro and the Sony FE 90mm f/2.8 Macro G OSS, both of which are quality lenses. For wide-angle, the Sony 10-18 is good. The Zeiss Touit 12mm f/2.8 is said to be even better but of course it doesn't deliver zoom flexibility. But to complete an underwater set, a fisheye is necessary if you want to really close in that water column distance and get the most striking perspectives. In my opinion it's there that the Sony system struggles with native options.
I hear good reports of Fantasea housings but have no personal experience. I have lots of Nauticam experience and although more expensive, Nauticam currently clearly provide a more mature and comprehensive system that is continuing to evolve and expand. That may be important now, but as your underwater photographic journey progresses, it may become even more important. For underwater photographic gear, post sales support is also important and you may wish to take availability of local support into consideration when making equipment purchase decisions.