You should probably start getting rid of all the crossing wood within the tree/shrub. Select other main branches to create a form you're trying to create. Those you'll keep of course. Then start pruning out other large branches to open it up.

All that spindly wood, get rid of that too. A lot of that will be the interior crossing branchlets. Removing this wood will beefen up the stuff you want to keep, to form your, look.

Dax

P.s. Pruning after the leaves have fallen would be better. It would also create less stress where it is now opened up and more vulnerable to insects and other pathogens. Ideally, winter-time is very favorable.

This type of long, leggy, even whippy growth is very common with young Sango Kaku's and is something they tend to outgrow with time and establishment. Generally the recommendation is to head them back to a similar length of the rest of the canopy or alternately, remove these shoots/branches entirely. SK has a vase-shaped habit so how much "opening" is necessary is a matter of some opinion and can't be resolved here easily without seeing the tree :-) I wouldn't do it just as a matter of course, but only if the growth/habit of the tree required it.

SK is so commonly and widely grown and planted in this area that there is some dispute about the quality and real provenance of many on the market. I see many that are short, squat and extremely shrubby/twiggy in appearance and would lend themselves more to Dax's recommendations than better quality, well-grown stock that already has a desirable form. FWIW, my own tree, now approaching 15 years, met with pruners this season for the very first time and that was only to lighten and lift a very full canopy - interior twiggy or crossing branches were virtually nonexistant. And because it already had some good size to it when planted, those long shoots it produced in its younger days were never routinely cut back......you would never be able to identify them now, as they blend in totally to the rest of the canopy.

In this climate, both early summer or early winter are suggested pruning times as sap tends to be least active then and less "bleeding" will occur. Summer is generally a better time for removal of those epicormic-like long shoots as they are produced rapidly with the spring growth push and easily identified....much less so when the tree is out of leaf in winter. I'd avoid late winter or much of spring as the sap is very active at these times and it is just too wet here, lending to the promotion of blights and fungal diseases.

In addition to maples bleeding when pruned in winter since this tree is planted for winter effect, if you cut it back during winter you have removed the part of it that is of interest.

Since the older bark of this cultivar is downright homely there is a case for cutting it back periodically and growing it as a shrub, without allowing a taller framework of unattractive mature stems to develop. This also prevents the younger red twigs from becoming a fringe or halo of red instead of a more conspicuous, tighter display closer to eye level.

Ingkind Those new long side branches will turn up on their own and reach for the sky, given time!
regards AL