@QuantumSparky - I'm going to try and help by going over some "guides", for styling bonsai.
The idea of bonsai is to get an emotional reaction from the image created that brings to mind something to do with trees. Some go for a more literal tree in miniature, some go more abstract. Bonsai really is an abstract representation of a tree.
Tree-ness: The big difference between a the impression of a tree versus a shrub is having a single trunk, and the trunk should have the visual impression of "thickness" or "caliper". In order to look like a tree the general guide is the diameter of the trunk is at least 10% the total height of the tree. So if the tree is 12 inches tall, the diameter of the trunk should be at least 1.2 inches, or 1.25 inches. Less than that diameter the visual impact will tend more toward "shrub" versus "tree". There is no upper limit on the trunk diameter. Generally trees less than 12 inches tall the practical upper limit is around 25% or 4 inches diameter. There are extreme "sumo" styles where the diameter of the trunk can equal the height of the entire tree. The extreme styles are more difficult to execute.
Bonsai trees have a tendency to be developed in thirds. The first third is the trunk. The second third is major branching and the final third tends to be twigs and leaves or needles. This is not an absolute rule, just a guide, artistic sensibilities always trump these guides.
To help with your nursery browsing. Bonsai from nursery material are usually brought down to bonsai size. If you want a tree that is 12 inches tall with a 3 or 4 inch diameter trunk, you need to shop trees at the nursery that already have 3 to 4 inch diameter trunks. These are usually found in the area with trees that are 10 to 15 feet tall. Yes, you take a 10 or 15 feet tall tree, and do a "trunk chop" to bring it down to maybe 6 inches tall. Cut to 6 inches most deciduous trees will back bud below the cut, allowing a choice of branches, one to become the next trunk segment and others to become side branches. This works well with deciduous trees. Conifers are more tricky because most conifers will not back bud if cut low to leafless stumps. With conifers you need low branches with enough foliage to support the roots when you chop the 10 foot tall pine or spruce down to 6 inches or so. No low branches, no low chopping possible.
But deciduous like maples, elms, hornbeam, serviceberry (Amelanchier) and a fair number of other trees will follow the above pattern.
Grafted trees. Generally for bonsai avoid grafted trees until you understand the ins and outs of grafting. There is nothing wrong with a well done graft of say a Japanese white pine, grafted onto a Japanese black pine root stock. On the other hand grafted Japanese maples can be very difficult to make into "good" bonsai because the graft union seldom heals well. Chopping trunks as advised above will often be below the graft union, which means the understock is what will sprout, not the named grafted cultivar. Often there is an upcharge for named cultivars, chopping means your throwing money away.
So only purchase trees that are not grafted. The normal species forms of trees generally work best, unless you know the tree was propagated by cuttings, rather than grafting. Junipers, boxwood, Hinoki (Chamaecyparis) and Thuja (American white cedar) are usually propagated by cuttings, so named varieties are okay for bonsai. Many shrubs are by cuttings. Ask at the nursery if you are not certain. They should know which are grafted (often bad), which are from seed (usually good) and which are cutting grown (usually good).
About your boxwood.
Nothing wrong with it except for the height of the tree, the trunk is quite slender. Below I have attached my red-headed-step-child of a boxwood. I say that because even though I have owned it for more than 10 years, I still can not settle on a style for it. It has never been styled. The diameter of the trunk is about 2 to 3 inches. the height is about 16 inches tall. This boxwood was collected from a 40 year old hedge. It took this box 40 years to develop the trunk but it was regularly pruned. It is possible to get a similar diameter quicker if you allow the boxwood to grow without pruning.
My suggestion would be to plant your boxwoods in the ground and allow them to grow freely, without pruning or other bonsai techniques until they have thicker diameter trunks. Or especially the 2 smaller ones you did not picture. Boxwood should be winter hardy in your area. They can be planted in the ground and forgotten about for a number of years, no problem. The one that is in the pot, if you really want to work with it where it is, let it grow taller. In terms of building a tree in proportions of thirds, your tree has a branch free trunk for about half its height. Let the tree get taller so that the first branch starts at about a third of the total height. That will give a better sense of proportion. Growing in a pot does slow growth. But you need to experience this "slowing of growth" for yourself. Put the 2 in the ground, let them run wild. Every year compare this one in a pot with the ones in the ground. The groundlings will catch up and pass up the one in the pot in a few years.
So I hope this helps. Do try to visit the bonsai nurseries in Pennsylvania, I highly recommend Natures Way, owned by Jim Doyle. Jim occasionally travels to Milwaukee to teach and demo, so I have met him and think highly of his skills. When you shop in landscape nurseries, don't look at height of trees, look at diameters of trunks. When you buy larger material to reduce down to bonsai size you will see much better results.
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