All trees thicken in key spots due to sacrifice branches adjacent to these key spots. If you wish to thicken a base of a tree while retaining some of the smaller branching in the top, then either a shoot low on the base will have to be developed or a branch grafted there, or the tree chopped shorter to increase the base due to low branching. All of these things take time and for all intents and purposes ruin an otherwise cool tree for a few years.
I relate building nice bonsai trees to the treatment of cancer. Like chemo therapy, the chemo therapy almost kills the patient to kill the cancer.
Likewise to make a bonsai beautiful, one must first make it ugly. That part is hard for some people to handle.
Building a base on a maple is not done in a colander by itself. To build a good flare or nebari, one must provide a place for the roots and trunk base to push against. This can be done by growing the tree on a tile or a wooden board. It helps to attach the tree to the tile or board by wire, staples or some other mechanical means. Seperation from the board will allow the tree to relax and stop its basal growth.
Your comment to back bud for basal growth is a good one and is needed to start that procedure. The problem is with maples either green or trident is the apical dominance of the species prevents it from pushing new buds from old wood in the bottom two thirds of the tree. This is only accomplished with drastic reduction of branches or drastic reduction in trunk height, which leads us back to ugly before beauty.....
Good luck, Al
