The same junk search terms turn up in all 40 accounts every single month

There are public lists floating around and I would treat them as a starting vocabulary rather than something to import. Half of any generic list is wrong for a given business - "free" is junk for most and essential for a freemium product, "jobs" is junk unless you are recruiting. What works is building your own from your own data once, categorised rather than listed. "Employment intent", "DIY intent", "education intent", each with the terms you have actually seen. Then when a new one appears you know which bucket it belongs to and whether it applies everywhere or only to the account you found it in. For the propagation problem, the master shared list is the mechanism, and the discipline is the hard part - the rule has to be that new negatives go into master first and get applied down, never added locally and copied later. The moment someone adds locally "just for now" you are back where you started.

Same problem at 22 accounts. What helped was doing the review in one sitting across all accounts rather than account by account. When you look at forty accounts one after another you see forty separate problems. When you look at the same month across all of them at once you see one problem forty times, and you fix it once.