How far back can I cut this massively overgrown panicle?

Moved into a house a few years ago - this hydrangea has to be over 12 ft tall, all the trunks are crossing over and seem to be dead wood (a lot of it bends and snaps easily). Hoping to do a hard reset to get it under control again but the trunks are pretty thick all the way up to about 6 ft. How far to the ground can we cut this back? From what I’ve read they can take (and prefer) a massive prune since they bloom on new wood but I don’t want to kill it completely.