Hi there,
There is a video I found useful on YouTube about maple bonsai-
https://m.youtube.com/watch?v=anyWlUK6E-k
Defoliation stresses the tree and slows growth and development of the trunk and branches which would be counterproductive to your goal at present (develop the tree you have into the start of a bonsai).
In short, don't defoliate.
Get a feel for the tree by peeking inside at the trunk and branches. Decide what style you want and start making moves to get your tree there.
My sugggestion would be to let the tree grow a sacrifice branch to get taper, select branches (as Shibui said to prevent reverse taper) and then feed and water heavily.
Branch pattern should be as follows;
First branch (right side of front of trunk)
Second branch (back of trunk)
Third branch (left side of trunk)
Follow this pattern all the way to the top.
You can vary the pattern by going left side first, back, then right. Be consistent. Some people also have the back branch as their final branch in their 3 branch pattern.
Obviously the growth patterns of trees don't follow rules so work with what you have got.
Go for an informal upright - it's the easiest style to master when new.
Good luck and I hope to see some updates.
Here's one of mine I'm developing. Yours is clearly more advanced!
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