I wonder if you could experiment a bit regarding the soil you use for the plants? A few years ago I put a balcony box on my balcony that was about four metres long and it took a lot of soil to fill it. We retrieved it with the wheelbarrow. We found a dredged spot of topsoil nearby and took it. It seemed to have everything in there, micro-organisms, worms and enough rotten to not need to take any more fertiliser. The plants we then planted in the spring thrived quite splendidly. In addition, we built a small wood storage shed as a compost collection point.

I wonder if more diversity stimulates growth. The lettuce and succulents seem to reflect that thought. Do with garlic what you did with the spring onions, the green stems are really delicious!

When we started a bed in my brother's garden this spring and scattered seeds, there were such a variety of species that began to grow quickly that we were able to watch it happen. It was a great experience! There was not even an infestation of one particular type of insect, the insects we saw were of different types and a natural balance seems to have formed there. Of course, a garden exposed to wind and weather is no comparison to an artificial biosphere, but I wonder how far you could push it. Presumably, however, you would need a slightly larger area and therefore more light and heat and again more equipment and so on. LOL - a condo has its limits, doesn't it?

P.S. Our observations led us to conclude that the competition for sunlight made the very many plants in a heap grow bigger and faster. I am almost certain that there is something to this theory.