How to prune nandina (heavenly bamboo)?

Mine has a scrub shape but then there are a few new tender shoots
sticking out from the top, it looks non-uniform and ugly.

I heard you prune back the old canes 8-12" off the ground, but what
would it do for the height? How can I get new growth to a uniform
height?

In my old house, I didn't know what they were, I just cut off the
stalks all top height wise, the top looked like vertical sticks for a
couple of years (real ugly) but after a couple of years it regrew
nicely.. maybe it took that long because we didn't have a sprinkler
for that area.. or maybe I cut off too much.