Here's my thoughts....my theory. I may be totally off.

I trunk chop now.... while the tree is dormant. Since the tree is well
established with an extensive root system.... there should be a lot of
stored energy going to a small area in the spring.... the stump remaining.
because of this im thinking there should be some prolific growth and the
tree should be strong because the roots have never been touched. After the
leaves harden off in late spring......air layer at the base.. with a large
ring cut and wire tie off.

I guess it would be better to let the tree go for a year to recover.. but
would it really need to recover?.. it seems to me that because the roots
system has not been touched.. and the trunk chop done in late winter while
the tree is dormant ....that the trauma to the tree would really be minimal
(rather arrogant of me to assume??? ). and air layering sooner than later..
rather than training the tree in the ground... would give more time to
start developing the nebari.

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