I am just a newbie so everything I say you need to take with a pinch of salt.
First of all I love your tree.
Regarding your method of pruning...it is wrong...
First as Enrico said You need to grow the lower branches...and for that I would follow WP technique.
You dont have any movement in the lower branches...by cutting back you will also achieve that.
What happens when You cut in summer is that:
You have already some growth on the branch...
in late summer the foliage stops slows growing and most of the energy is directed from that point on to storing food in the roots.
If you have longer summer you will have a second flush of growth, which will be much finer...(well ramified)..with shorter internodes.
Never trim this tree during winter or you will have explosive growth in spring.
since you will have food stored in the roots and all that food will be expended oh growth that is much less due to the trimming.
What You can do is let the roots expend their energy in spring into producing shoots and leafs....then you can trim hard in order to get slower much more refined growth...and let the tree grow...so You get the girth you need in your branches...dont keep on pinching it because you will never get the thickness of the lower branches that You need.
Ones you achieve that thickness..and movement...You can proceed with secondary branch development...ramification and last leaf reduction.
How is your nebari? In bonsai always start from the bottom up...develop a good nebari and trunk....and then start the rest.
Maple can have magnificent nebari...I hope You know how important that is.

After looking at the picture...again I noticed...the top most part has very little movement and tapper...I would reduce the top if this was my tree...
Your tree has lots of potential...do it right.
Do you have some reverse tapper or it is the picture? Remember bar branches create that.

The tree has no reverse taper.
The nebari are short, but quite fat. The other side of the tree has more defined nebari. I am planning on letting the bottom branches grow out and thicken up a bit.

Great potential on this tree!!!
It is à bit top heavy though. I would definitely thin out the top, also to promote more and better growth on the loser branches.
Looking forward to the progression.