Ok...
Sharpes Pigmy is a really wonderful cultivar. The leaf color in the fall is spectacular! It can get very nicely ramified with lots of little twigs. There is great potential here!
The thing about Japanese Maples is the cultivars vary as to how well they respond to cutting back. Some will only send out a single flush of growth in the spring. If you cut back, well, it's done. Others when you cut back respond by popping new shoots as the base of the existing leaves. Much like a JBP responds to decandling.
I don't know how Sharpe's Pigmy acts. Sergio,
@MACH5, does. He has one. You should get him to guide you on this.
The fancy cultivars, like Sharpe's Pigmy, tend to be weaker than regular old green "Japanese Mountain Maple". The plain old JM make great bonsai easily because of its vigorous growth. The fancy cultivars are slower.
If this were my tree, I wouldn't do a thing until after fall color has started to fade. Then once most of the color has faded, cut off the leaves, and that's when you do your major cut backs. When the sap is falling to prepare for the winter. Of course, seal the cuts.
But, please contact
@MACH5 before you do any cutting.