I would choose a grow box rather than grow in the ground. Julian uses 100% Turface as his soil medium. A slightly coarser soil mix is what I would recommend. I have two of Julian’s airlayers, and they have really thrived once I moved them into APL. I do use Julian’s micronutrients on all my white pines every spring.
About your trees: I see some really long internodes. They’re too long to be used for bonsai. Rather than simply cut back, I would take airlayers since it’s Zuisho.
A warning about Zuisho: it’s a dwarf cultivar. But, at the same time it’s a vigorous cultivar! This combo means that sometimes it wants to make thick branches. That is, branches seem to thicken, but the trunk does not. If that occurs, your tree can look disproportionate. So... be vigalent about keeping the branches from thickening too much too fast. Keep them cut back.
And plan on wiring and removing wire often! Because the branches thicken quickly, wire cuts in quickly. If you want to minimize wire scars, you will have to keep watch on it and remove it as soon as you notice any cut in.