You can put this into something smaller, and boxwoods do deal with root pruning pretty easily. I don't think I'd go too shallow or small yet, I'd give it some room to grow. But from the sounds of your soil, (what is Nappa?) I think I'd repot, and you could do a smaller trainer instead of a box at this point if you want. From what I've found with boxwood, they can do pretty shallow final containers.
I think if you use the first pic in the post as your front, I'd showcase that wonderful curve of the front of the trunk that goes left then so gracefully back right. Fan the branches out from there, to complete that line...