Taper
To create a tapered trunk over time, you would identify a new apex (a branch of smaller diameter) and repeat.
Start lower (closer to the primary branches) so the taper is progressive. Depending on the desired final height of the tree.
While a straight trunk may not be desirable except for a formal upright, you can select the new apex that is directionally most upright.
You bend a branch to vertical to replace the trunk you chop off.
This might also involve bending the trunk at that point (using wire) to get the branch vertical.
You pick a branch facing the front of the tree so that the scar of the trunk removal is at the back of the tree and hidden by the new leader.
formal uprights DO have taper.