How are these bad actors coordinating hashtags in a way that can't be detected?
If they really have some sort of secret channel that no one else can infiltrate, why not just use that to distribute stuff directly instead of running an evil cabal of hashtags?
Surely, even given the evil cabal of hashtags, you could find a lot with simple heuristics? When you ban a tag, track which new new hash tags suddenly explode in popularity. Pull a list of hashtags with an unusually high number of links/images to sites that don't normally get linked? Track the people who follow these hashtags, and see what hashtags they go to next
If you actually want to fight the problem, it doesn't require a huge amount of resources - the problem is entirely that Meta has no actual desire to fight the problem (after all, it costs money and doesn't produce any benefit to them)