Well, I'm very happy to hear that you don't want to use herbicides to kill your grass. That is the first step to a vibrant healthy garden. Good on you!
I realize 60X40 is pretty darn big so, a shovel that I normally recommend wouldn't really suit you here unless you have a whole day to work at it. Unfortunately your best bet is to rent a rotatiller and till the area (only once) and then, go over the area with your shovel in hand turning in anything that pops up in the future.'
Back in the early 1930's my Great Great Grandfather would walk his fields (6 Sections, obviously not in one day) and hand pull any thistles and what not that he saw. So, this is very doable.
Growing Garlic is dead simple, just plop an untreated clove into the soil and cover it up. Be sure not to overwater.
Be vigilant with your shovel and you won't have to till again. Tilling is actually really bad for the soil because it shreds beneficial soil organisms and breaks up the organic matter to much so that the nitrogen, phosphorus and Potassium (NPK) is used up to quickly. So, doing it once to get started is fine but if you till on a regular basis you will end up with either sad or clay which, is not good.
Anyway, enough on that.
Each fall, add mulched leaves and manure to the area as a 2 layer sheet mulch. This will add nutrients to the soil and your garlic and other veggies will grow better. Also, look up Garlic or Alliums and companion plants on google to see what plants that you can grow and sell with the garlic that will detur insect herbivory and benefit the garlic in other ways.
Don't use synthetic fertilizers on your crop because they will break down soil structure and your cros won't do as well. The addition of leaves (Not Walnut) and manure each fall will provide your plants with all the nutrients they need.
So Rhubarb is a very hardy perennial that you should plant on the edges of your garden or plot. Each fall cut it right back to the root and it will come back in the spring. I'm sure you already know this but, only the stalks are edible the leaves are very rich in Oxalic Acid which is poisonous to humans. However, they are great in the compost pile or in your sheet mulch as a green.
I hope that helps.