I'm noting that surviving box was cut on top and bottom equally. Which fits into what I've noticed works.While I only have 7-8 boxwood and am no expert, I have had really bad luck repotting after June. One died and another is finally recovering after two full years. It will be fine until spring when I’ve had my best luck. I’m happy I documented my oak style broom boxwood here because it has been the best responding to what I’ve done (until recently). Check it out, it’s a similar story to what you have. I’ll be updating it soon. https://www.bonsainut.com/threads/a-blue-bill-buxus-built-to-be-a-broom.25523/
I reckon you may have just cut em a bit early.tried trimming the boxwood in the link July 1st and it has been super slow to push new growth. It’s finally growing again a month later
Not a bad plan at all. What does and doesn’t work for me might or might not work for you. I will recommend that you leave yourself a lot more time then you think you will need to repot. Those roots are brutal to comb through in that state as well as not scarring up that thin bark trying to get at the bottom roots. I’m thinking it could take you up to six hours if haven’t done many root bound repots. It might only take you an hour if you cut the bottom off close to the tree. I think it took me 3-4 hours by the time clean up was finished and I was exhausted. I got a tonne of enjoyment out of it though. Good luck and have fun.Thank you all for the advice. I got two of them, I just like the roots on one better than the other. So I guess I shall experiment a bit and repot one now and the other in spring. We shall see how they do!