Covers all main points, thanks for sharing, Luke!

One point I'd like to elaborate.

Salt-spray.

Any garden plants need to be tolerant of salt-laden spray, even if otherwise well provided with rainwater. This limits you to hardy halophytes (salicornia, sea celery, warrigal greens etc) or requires an enclosed greenhouse able to survive gale-force winds.

Fresh water is also certain to come with a dusting of salt spray. In a high-rainfall climate this would hardly be noticeable, but in drier climates it may leave rainwater non-potable. Mechanical diverters will be confused by wave action, so probably requires electronic sensors, or electrodialysis of the captured rainwater (much less energy intensive than desalination, but still another complex system to depend on).

Photovoltaic panels do not appreciate spray either. Dry salt deposits are broad-spectrum reflectors of sunlight, so even a modest salt build up on the panel will significantly degrade it. Regular cleaning a necessity in periods without daily rain, and this requires designing for safe and easy access to the panels.

The chloride content will also disrupt the surface anodisation of anything aluminium, need to use more expensive 316 stainless on anything exposed to spray.