How sure are you that it's a Hawaiian Schefflera? It's hard to tell from the pics but that kind of looks like it might be a species that tends to be a bit skinny.
-The "petioles" are the leaf groups that come off one stem and you can kind of think of each as 1 leaf.
- The stems on new petioles tend to keep growing longer if the leaves aren't getting enough light.
-When cutting a branch back, new growth will usually start at the last remaining node from the position of the last petiole or petiole scar and new branch growth will grow in the direction the petiole was facing.
-To get a thicker trunk you are going to want to encourage back budding lower on the tree by chopping off significant portions of the newer growth. This will also have a chance to increase branching.
If it is a Hawaiian you can encourage aerial roots to form by keeping it moist. They tend to like to drop from points that are more horizontal. This can help give the effect of a thicker "trunk", and help get a tapered look.
I suggest you cut the top half of the tree off, get the cutting to root in water and then plant it right next to the existing trunk and continue this until you get the effect that you are going for. The cuttings root very easily.
Here is my Hawaiian Schef project, it's in a ugly phase now but you can take a look at how much the trunks have been hacked up and see if you like aerial roots.
To give an idea of scale, that is a 15" diameter pot.
It's a really fun plant, very forgiving, fast growing.