I reckon early spring to root prune hard is best in saying that I just repotted one last week and I reckon it will be ok and i removed 90 % of the roots . Remember if u root prune a tree it takes a while before it can start taking up water again properly so I'm
Keeping mine well watered and in the shade and slowly bringing it back into full sun over the next few weeks.it is getting hotter so treat it like a cutting . If it were mine I would saw the bottom 2 thirds of the root ball off shake all the soil from it and arrange the roots plant it on a cd in a large shallow pot in a loose fast draining soil
Mix and let it rip until that trunk is at least the size of my finger and then chop it all off leaving Only an inch or 2 of trunk then grow a new leader and develop from there. try and end up with a tiny little tree about 6 inches in height.
All the growth you remove could be turned into cuttings .
But hay I have only been doing bonsai for a year and haven't developed jack s.
So don't take my word for it .'
I wonder if any one agrees with what I said ?