I think people don't like cooking at home because they don't season things properly and then are stuck with a lot of food they don't like. And even if they do, they've made a large amount of one meal and get burnt out eating the same thing for three days. To have variety seems to take cooking full meals every night, which is a lot of work.
You have to think like an industrial kitchen. A restaurant doesn't cook every meal for every customer 100% from scratch as they are needed. Not even the highest of the high end restaurants do that. But they also don't have vats of fully prepared food sitting around to get gross and dish out at the last minute.
You are similarly one person having to cook for 21 people every week: you, 3 times a day, 7 days a week. The ways you would make that work for cooking for 21 people in one meal are the ways you make it work for cooking for one person over 21 meals.
The key is to break a set of common recipes down into key, bottleneck components that keep well. You cook them 80% of the way in bulk, then cook them the last 20% when you need them.
You can also combine some of your prep-time per meal into your prep-time for the week. It takes no more time to cook 6 chicken breasts in the oven as it takes to cook 1, but if you want chicken breast for dinner tonight, you might as well cook 5 more. Then the next day you can slice it thin to have on a sandwich for lunch, dice it up and mix with spices for tacos, mix with mayo for chicken salad, put on top of salads, etc.
If I'm chopping onions, I often only want half the onion, but I will chop the whole onion and stow the half I don't need because I'll probably need half a chopped onion the next day, too. Rice takes about 15 minutes to make, regardless if it's 1/2 cup or 2 cups. I can only eat 1/2 cup in a meal, so I make 2 cups and stow the other three servings.
When I make tacos, I usually make a LOT of ground beef with taco seasoning. Don't buy the packets. Taco seasoning is just spicy cornflour gravy. You make a spicy, erh, soup with whatever ingredients you like and then thicken it with cornflour. Now you're never out of taco seasoning. And taco meat goes great with eggs in the morning.
The next time you're at a restaurant, pay attention to the side dishes and see how many of them are either the same or are different variations of similar components. There is huge variety from few inputs in permutation.