Typically you want to do repotting/root work in the spring before the buds have pushed. After bud push the tree has already spent a lot of energy making new solar panels. I plant them to the soil level in the container. I was actually looking at my magnolia this morning. It’s recovering well from the initial chop, so I’m going to do a second chop soon. I was super conservative with the first chop and left most of the primary trunk. I’ve got buds all along the base now. When those mature a bit more I’ll be good to chop the rest.
Local climate is pretty humid. Not too much rain. If anything not enough. 