Some people have posted in the past that 6" depth with solid bottom is too shallow. Even the shallowest rooted plants grow deeper roots that 6". With solid bottom, the roots will have no option but to grow sideways but you are planting closely with square foot gardening method.
I sometimes refer to square food gardening, and I do plant closely but for the most part, you really need to consider how wide the plants really grow -- too close spacing would mean they are shading each other and there is inadequate air currents, in addition to how their roots may be robbing nutrients and water from each other.
I think I sometimes get away with it by interplanting different growth habit plants.
As far as bottom of the bed -- if you have no soilborne issues like bacterial wilt or root attacking nematodes, or invading tree roots or contaminated soil, almost any kind of soil will improve over time, and the plant will help to do so by making their way into the underlying soil -- as long as there is soilfoodweb interaction between the underlying soil and good soil above. I do use cardboard or 4-5 layers of packing paper for building a new bed -- this not only suppresses underlying weeds and lawn grass, but also invites and attracts earthworms.
I think it helps to grow really deep rooted plants in rotation with shallower rooted plants, and root crops, with potatoes and sweet potatoes that require thorough digging every few years.
Hardware cloth is recommended against voles and ground squirrels, but I wouldn't even use that unles you really really have to because it would interfere with working the soil as well as some thicker rooted crops.
My practically solid clay subsoil has been becoming richer and deeper -- it's so satisfying to feel almost no resistance when I push the garden fork in to aerate (push the garden fork in to depth of the tines, then rock and lift but not dig up) -- I used to have to stand on the fork, then give a good one foot step, now, I can just use my arms without hardly any weight behind in my best beds.