Apologies for not being clear enough. Definitely cut down, comb out roots before growing on in a grow box (or any other growing space, ground, grow bed, etc)
Hard to tell what you'll find when you get the soil off but frequently lots of circling roots, sometimes close to the trunk from when it was in a little pot years ago. Sometimes we can bend some out straighter but often the only option is to chop lateral roots real short. Chopping short is often good because that forces the roots to branch out (ramify) which is much better nebari than long, straight roots.
Flatten the root system is also correct. Again, some thinner roots might be able to bend to be horizontal but I usually just chop any vertical roots off close to the base of the tree. After the initial hard chop the trees usually put more effort into the surface roots and don't bother growing big down roots any more.
Japanese maples can tolerate quite severe root pruning. I don't usually go quite as hard as tridents but you'll probably be surprised how much can be reduced and still survive.
Not sure I have any radical root reduction pics for JM but here's a few tridents I cut back.
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Found a few 'after' JM root reduction pics.
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The roots above were long, straight roots and were chopped where all the smaller roots are now. You can see that new roots almost always emerge from the chopped ends so it's important to cut long roots right back where you want new roots to start. Also note that thicker roots usually produce more new roots than thinnest ones.
Pics are 1 year after root chop.
I cut all those again after the photos, just leaving 1/4 - 1/2 inch of the new roots to further ramify the root system as a whole.
Definitely harder to make that call with little experience.
I would definitely do it down here with leaves at that stage but maybe lots of experience makes that safer for me?
Most would say play it safe and wait till next year.
I guess it might come down to how much of a gambler you are?
