Most modern pruning advice suggests you can do small-scale maintenance pruning at any time.
As for what to prune, what do you mean by 'out of control'? Personally, it looks pretty good to me, if a little dense.
Your tree is a dissectium japanese maple. They are supposed to mature to a graceful, slightly weeping habit, with the forms of the branches slightly visible through the leaves.
Here is an example of a similar mature tree (this one is red, but should have a similar habit)
(from here: www.thesitegardener.com/master_images/2221/original/Acer_palmatum_var._d._Baldsmith_1.jpg)
Unfortunately, yours appears to have been grafted to a straight trunk a few feet off the ground, which is a short cut growers take to make them look taller quickly. It's always going to be a little awkward, but it can still be attractive.
You do not want to shear a tree like this. Don't cut branches or shoots in the middle, always remove them where they attach to the parent branch. You goal should be to thin out the dense and crossing branches in the middle and let the longer main branches dominate. Any branch that is growing back towards the middle of the tree, or rubbing against another branch should be removed.
Go slowly and take time to step back and look over your progress. Absolutely don't remove more than 1/3 of the leaves in one year. Probably much less than that should be a good start. You can always cut more later, but you can't put a branch back on.
Or, if you prefer something more geometric and manicured looking, rip it out and buy a boxwood or a standard rose or something that suits your taste. -- I know that sounds flippant, but I'm serious. Life is too short to hold on to plants that are never going to be what you want. You're only going to feel more guilty as it gets bigger, and waste time growing the plant you do
like in the meantime.