So lets break this down into steps you want to take and why you may or may not want to perform those tasks right away or in conjunction with each other:
Operation 1: Repotting
Operation 2: Trunk Chop.
Operation 3: Build taper, over time.
Best possible sequence of events for these would be the following:
Opearation 1, Operation 2, Operation 3. Your roots need to be growing healthy with room to extend before performing a trunk chop. I am sure there are people that have success at doing both at once, or in close succession, but I also think that's with full attention paid during aftercare and a vigorous and healthy-growing tree to start with. You don't really know what kind of care the tree had before you got it, and if it is as severely root-bound as it is, my guess is that you need to build a bit of vigor before the trunk chop. Here is what I would recommend:
Operation 1: Repotting- Do this next Spring. As soon as you see the buds swell, repot the tree. Maples are pretty hardy and can have a good amount of their roots removed. This helps both points 1 and 3. Successful repots of Maples really give you the best opportunity to build good, strong nebari. Be sure to plant in a pot that is shallower than you would typically use, but also wider than you would typically use. A nice, shallow, oval is a typical choice. Keeping the tree in a shallower pot forces those roots to spread out, which, in turn, increases the width and strength of the nebari.
Operation 2: Trunk Chop- Do this in Spring of 2019. Again, time the chop for bud swell, and all of the strength that was being directed to all the buds will go into those that you are starting the next step of the trunk and first branch with. In 2018, be sure you fertilize enough to pump as much vigor into the tree as possible. This will prepare the tree for the trunk chop in 2019 and set it up to explode with growth from the buds you cut to.
Operation 3: Building taper over time- Taper is always the struggle right? Not with maples you are trunk chopping. Each time you do a new chop, you build another step of taper. Think of the tree you chop in 2019 as the starter. then after letting it grow out the entire year, reduce to two shoots, training one as the new trunk step, and one as the next branch. Grow that out for a year or two, depending on how much of a difference in taper you want, and do the same thing again, adding one new trunk step and branch each time. And before you know it, you have a pretty damn good looking maple. Over the course of 5-10 years, but thats nothing in bonsai. Most important part is letting them grow out like crazy after each operation, while keeping the already developed portions heading toward the refinement phase. I always suggest Peter Adams book on Bonsai with Japanese Maples. It's simple to understand, great illustrations, and examples of his trees. You get to look into the brain of someone thinking about maple design. Some of the horticulture stuff is a bit out-dated, but if you read for purely design understanding and how to build design over time, there is a wealth of information.
