Thanks for the help.
I wasn't getting messages from iOS about low storage, but I was from my other programs that require disc space, like After Effects and Maya. They didn't want to open at all.
I already have all my hidden files visible (runnning that script in Terminal is one of my basic setting up tasks that I always do)
so there's nothing surprising or weird living in my Systems and Library folders as far as I can see. The numbers just aren't adding up to what the storage manager says is on my drive; it's maddening.
Is it really possible that iOS was storing 300G of temporary purgeable files in some hidden corner of my drive? That seems ridiculously excessive. Like I need a defragger excessive, which is I think what first aid did to resolve the problem, but the problem is recurring. Currently that "systems data" category is back up to 93G.