Great looking boxwood!
When determining where you want to go with a tree, it's helpful to ask a few key questions:
1. Where in the development of this tree am I? Primary/structure, secondary/branching, tertiary/ramification?
2. What style does the tree want to be naturally? What style am I looking for?
3. How big do I want the tree to be?
4. What work has already been done to this tree?
5. What timeline am I working with?
Based on what you've given, I'll try to answer those questions first and give some perspective on what they mean:
1. You're in primary development, so setting the general shape is the primary objective. It looks like you have already done that! You might consider some structural wiring in the future as well to help open up the branching structure, but for now the pruning you've done is probably sufficient. Secondary development would be setting secondary branch structures and some level of refinement, whereas tertiary development would be focusing on ramification and refinement of the tree.
2. Given how you've pruned it already and the branching structure, broom style seems to fit well here. So given that, you'll want to work to keep that going over time. If that's not the style you're going for, you'll want to re-evaluate how to get there - does it need to grow more? Wire? Do you need to remove more branching or change the angle?
3. Looks like a fairly small tree, so if that's what you're going for, then great! If not, you may consider planting it in the ground or growing it in a much bigger pot in order to let it run. You can plant it on top of a tile or paver to keep the root structure shallow, but let it run into a bigger pot or in the ground to speed up the growth process. If you want the tree to be smaller, you will probably want to remove growing tips to induce back-budding so you have branches to cut back to over time.
4. It looks like you've done some pruning in addition to the root work. Given the 40-50% root reduction and the pruning you've done, I would wait until next year to do anything else. Boxwoods are broadleaf evergreens, and given their evergreen nature tend to behave more like conifers when it comes to speed of growth (i.e. slower than deciduous and tropicals). I believe that most people would typically recommend to do a max of one major operation (root work or pruning) in Spring and one in Fall for nursery stock. Collected/older material would reduce that to one major operation per year.
5. When do you want this show-able or in a bonsai pot? Ultimately, this helps drive how aggressive you need to be with your styling, etc. But you also have to make sure the timeline is realistic. If you move too fast, you risk killing the tree or setting it back by several years and having to nurse it back to health.
Once you answer those questions, when you do further pruning work on the tree, consider removing any "flaws" in the structure first (within reason). This means removing bar-branches or reducing areas with 3 or more shoots down to 2 to prevent inverse-taper, removing crossing branches, etc. Pretty much all of the standard "rules" that you'll find about bonsai. The earlier you address the flaws, the faster you can move on to the other stages of development. I would always caution to do so "within-reason" to always put the health of the tree first. If you can horticulturally keep the tree growing (new foliage, new roots, making it bigger, etc.), then you can work on the art side of things.
One final thing to consider is time of year + species. We're running into summer, so heavy work on Broadleaf Evergreens isn't really advisable. You'll want to wait for Fall or Spring to do that work. Given what you have already done, I would work on the horticultural side of things until next Spring, and then re-evaluate based upon how it's growing.
Hopefully others will chime in here too. Happy Bonsai-ing!