I will only comment on the self-watering aspect. You may be very surprised by how much water that volume of soil consumes when allowed to suit itself. You may find that if it rains enough to keep that meager reservoir filled, it's raining enough to do the watering directly.
I've done self waterers built from oval and round goat size stock tanks that used 1/2" nylon rope (which works very well) to bring water from the bottom part of the tank that is roughly the same volume as the soil. It empties the reservoir more quickly than you would think, even with that ratio of soil to water compartments.
I also watered lines of puts from a 4" PVC pipe that ran the length of the line, beside and below the pots, nylon ropes run from the pot soil into holes in the top of the pipe, filled from an upturned "U" at the end. I would guess it, too, was about equal volumes of soil and water space. And being pots, which consumed more water, it needed very frequent topping off.
And water generally must wick up. At any time a wick has any portion of its run lower than the top level of the water, it will flow down the wick continuously and drip from the low part of the wick, not wick upward according to the soil's need.
I would consider designing so that the soil compartment sat above part of the water reservoir so that all wicks ran strictly upward. If the reservoir and soil compartment could also be one unit that can hold water, with a permeable barrier to support the soil, the ropes wicks can be run up though the barrier, the water catchment can be above the water level outside the box, and a small hole just below the bottom of the soil level will keep it from flooding in heavy rain. The hole just becomes an overflow when the reservoir is full, either from the catchment or from rain soaking down through the soil.
That can also give you a larger reservoir, and there will little or no evaporation because it's fairly isolated from the environment. Oh, and you want some way to gauge the water level. A small pore pipe down into it where you can stick a dip stick is enough, or a stick with a float, if you want to see it from a distance. The same pipe can be the filling intake when rain is short.
One of my future projects is to do one where the soil compartment fits down into the outer walls, with the water in the bottom, or is a box that sits on top of the same size water box. The ropes will just hang down into the water when the box it set back down. Major maintenance is kind of awkward right now when you can't lift the soil out. I haven't worked out what to make all that out of, so I'll probably shape the removable soil compartment to fit down inside the stock tank to rest on stops.
