You’re right that the 30% collapse of the US money supply was the proximate cause of the Great Depression, but Fed policy didn’t create the collapse.
Friedman and Schwartz hold the Fed responsible for failing to act once the collapse started feeding on itself but not for causing the banks to fail in the first place. The absence of FDIC and America’s peculiar branch banking law played a large role in making the Depression worse in America than the rest of the developed world.
Smoot-Hawley passed after the Depression was already well underway. And while it didn’t help the situation it wasn’t much different than the Fordney-McCumber Tariff that had been in effect for most of the Roaring Twenties.
The Fed should have provided liquidity to the banks, and that would have prevented most or all of the failures. But they raised the discount rate and made it more expensive for banks to borrow.
I've read that the Fed thought they were defending the gold standard, and that might explain their actions.
Irwin’s book says that with Fed deflation Smoot-Hawley was a 30% hit on the economy. Still lots of debate about the timing and effect on the Crash, which is far from solved. Main point is, S-M was not needed. American industry by then was doing fine, and only Germany had put in place their own tariffs.