I would NOT trunk chop this tree. There's a pretty good skeleton for a flattop there already. I'd shorten all the top branches by 2/3 or more. I'd saw off the bottom 2/3 or more of the root ball then take a hose to wash out all the existing soil. Then choose appropriate roots...That upward turned knuckle emerging from the soil between surface roots is a prime candidate for removal. You're looking to create flat root mass with untangle, unconfusing roots that emerge from the trunk naturally.
I'd also remove all those bottom shoots and reduce that lateral branch you've wired to within an inch of the trunk. That reduction will give you a base to work a branch from. It is a mistake to think that flattops have all their branching at the top of the tree. Older BC that have become flattopped tend to have older lower branching, just not a lot of it. The way BC works is they grow as pyramid triangle-shaped trees as youngsters. The old they get, they push more growth at the top, which overshadows the lower branches. Lower branching (a lot of it, or all of it, depends on the tree) dies off. Some older lower branching can remain, but it's often busted or not dominant. Lower branching can be incorporated into flattopped bonsai BC.