These are often treated as spring cut back shrubs. I'd wait until next spring and then cut it back, and do that every spring,

sounds like you are trying to force a VERY LARGE plant.. into a very small spot ...

do whatever that need be done .. and dont worry about it ... if it eventually fails for whatever reason ... your base problem will be solved ..

ken

Regardless of timing which I know nothing about, I wouldn't shear it, which makes me think hedge trimmers, since that damages leaves and so ends up looking unsightly with a large-leafed shrub like this. I would cut individual branches.

Around here, these are often winter-killed to the ground and then regrow to 6' or so. I can't imagine trying to keep it smaller than that. If that is too large for what you want, I might look for another plant or move this one to where it has more room. They regrow quickly, and as Ken said they want to be large.

I suppose you could tip-prune it to even out the growth. No hedge trimmers, use bypass pruners. I would say that you could knock 6 inches off the tip of each branch with great success. Take the clippings and try to propagate them.