Do you use https://meta.discourse.org/t/discourse-ai/259214#p-1260611-ai-spam-detector-5 ?
Stop Forum Spam Plugin is not official but it has been very, very efficient on my forum, including large attacks like the one you’re experiencing.
Also, if the bots post very similar messages, perhaps try to see some expressions or links they post that you can add to watched-words to automatically silence the users? I’ve never used this feature tho.