... which will be completely useless if they use a tumbler. It's only a matter of time before feds become skilled techies. If they currently have anyone remotely capable on their staff -- and let me remind you, they somehow managed to acquire server images of Silk Road even though it was operating as a Tor hidden service, so apparently they do -- then those technically-capable staff members will strongly advise them to tumble those coins.
Which service will tumble 32 million worth of coins? And if they do them per bust then people will still see a transaction of x amount leaving their wallet.
Exactly, regardless if they tumble it, they still have to tumble it. Which creates plenty of technical and legal challenges. I personally doubt they would essentially launder their own seized money for future operations.
They aren't going to tumble all 32 million. Even 10k-20k USD worth of coins is enough to work with for most criminal cases.