Four Oaks, my friend grows lots of lettuce at her farm and I can tell you what you need to know about bagging baby salad greens. You have to wash and spin the greens before you bag them or they will be unmarketable. It's a bit of a process.
We seed the salad mix pretty densely in a two inch band. For your situation, choose the row that gets the most shade if possible, as in your cooler and moister spot. Lettuce needs lots of moisture or it will bolt and get tough and bitter. Possibly cut and come rows of lettuce might work better for you, as the regular cutting seems to keep them from bolting as easily as well. The better you weed it the faster it'll grow. Maybe a light coloured sheet mulch would work to minimize the weeding.
To harvest the cut and come again lettuce, cut with a knife or scissors at least 1-2 inches from the base. This allows the center leaf to remain intact and the side leaves/partly cut leaves also help it to regenerate faster.
Step two, you need a big tub of clean water. Dump your cut lettuce into that for a minute or two. Swish it around by handfuls to take off any dirt, then pile it onto a drainer tray to let most of the water run off. This process also gets rid of any slugs, aphids etc and gives you an opportunity to pick out any weeds that snuck in.
Step three, we have a big industrial size spinner at the farm but any salad spinner will do if you're trying it out on a small scale. (Another friend got stuck doing commercial lettuce with a home spinner, I hear it was extremely tedious.) You pile the lettuce in, turn the crank and let most of the water run off. The spun lettuce is then transferred to a clean tub ready to bag.
Step four: adjust mix as desired into the bags and weigh on the scale. There should be some air at the top of the bag and enough of ends to tie it shut. Hold the two edges of the bag in your hands and spin the bag around, then tie the two ends in a knot. This keeps air inside as well as moisture and allows you to 'fluff' the lettuce and keeps it from being crushed.
Pack your bags into a tub and put it in the cooler, ready for market mobile.
I have some kale and lettuce growing in about the depth of a 1020, had 3 or 4 cuts off it already but we're talking about a 2 inch harvest - itty baby greens. I have more growing in a tub twice as deep which is twice the size (took a week or more longer though) .
Baby Kale is good stuff, grows about twice the rate of lettuce too. Your mustard will probably be the fastest.
Here's my 1020-depth lettuce and kale: