I wonder why they started with lettuce. I get the nutrients and minerals part, but the calorific value is low. Potatoes also grow well using hydroponics and would seem to be a better base choice for longer orbits.
Lettuce, especially a hardy cultivar, is delicate but fast growing and parasite resistant. And not picky about the growth media usually. Potatoes are slow and I suspect they might start to rot on the space station before they got to full size. Hydroponic potatoes is possible but tricky. But I'm only making a guess from having grown these plants here on Earth.
In regard to calories I've heard good things said about oats and switchgrass. Not many people's favourite staples but they grow like weeds under wide climatic conditions with minimal water.