While mixing 250 million dollars is near impossible in the short run, you could easily mix a million dollars a month every month for the next 10 years. Mixers aren't a full proof way to obscure funds but the FBI has outright admitted that it's nigh impossible to track it down.

Furthermore there are decentralized mixers that run on Ethereum so it's no longer possible to go after the operators of mixing services. tornado.cash has billions of dollars available to mix with which is more than enough to obscure millions of dollars worth of ETH at a time.