Jal, you are right about all of that.
Sitting on 4" thick concrete bricks added that many inches to the 20" of depth in my 3'x8'x20" beds. My husband didn't think they needed to be that deep until I told him (then showed him) that carrot roots can grow as deep as 5.5 feet. Onions have a lot of shallow roots but some deep ones as well. I handed him my book on plant roots. He was quite surprised. The two 4'x8'x12" beds were set up to use up the extra soil we had. We plan to go up another 12" this fall.
As you noted, roots do travel in all directions and take up a huge amount of space. Square foot gardening is OK for some plants but you can't crowd plants or they won't do well. If you study the root developement of individual plants, you won't plant them so close. I planted my broccoli, brussel sprouts and cauliflower at the spacing recommended by the seedsman for an in-ground garden. Same with the onion family, beets, spinach, lettuce.
I did however, decide to show my husband what happens when you plant too close. So some sacrificial plants in one area. It surprised him to see how healthy the well spaced plants are versus those that are too close together. As well, the bugs can hide better and do more damage than had we been able to see them.
I also agree with some of the soil mix. One of our garden centers offered a mix for raised box beds. When I got the general percentage of everything in there, I knew I wanted to make my own - and did. I got the best topsoil possible at a really good price - well, if you have to pay for it. I also used good local compost and well rotted manure from a local Vermont company that I would trust with my plants. My beds hold enough moisture that I don't have to water every day and drain well so nothing sits in water. I haven't had to fertilize the raised box beds.
Where I push my plants to grow for me is in the containers and I do still use them but not the small ones I used when I first started.
If I had the space and decent soil and a much better back & knees, I would love to have a garden like yours, Jal, because that is what I grew up with. My Dad had 9 kids to feed and we all helped in one way or another to maintain that huge garden.
Like many people across America, I am very much into growing as much of our food as possible so I know how it is grown. We don't all have the perfect soil or space to garden and so we look to the "experts" who will sometimes sell us what we don't really need. We also have a few of us older folks who love to garden but our some of our body parts rebel.
I think we could be respectful of the different methods of gardening because each of us is doing it the best way we can/know how. Many are first time gardeners and we don't want to discourage anyone from gardening or visiting this website because we have our own way of doing things.
I'm certain every gardener across America would love to have huge gardens and much longer growing seasons, better backs & knees and the experience of many years of gardening. Many are just learning and I say Kudos to everyone who is a first time gardener or trying a new method or plant.
And for all the experienced folks, I say thank you for sharing all of your wisdom. You do indeed inspire us all.