That’s a myth. You are probably speaking of Calvin Coolidge and Herbert Hoover who were fairly laissez-faire conservative Presidents leading up to the Stock Market Crash of 1929 and the subsequent Great Depression. Both events were not necessarily related as many schools lump them together.
Hoover actually tried fixing the problem with more government solutions before FDR came in and really muddied up the water with even more government solutions and regulations. Both Presidents actually exacerbated the problem instead of letting the market auto-correct itself. Eventually deregulation by Congress pulled us out of the Great Depression but the damage was done and we have never seen the end of these awful federal programs that came about from this time.
I’m not sure what you are referring to about isolationism because FDR was in the White House during that entire stint until he died. Hoover may have seen some of the beginning transgressions, but America’s isolationism wasn’t a bad thing at the time considering we were broke, just fought in WWI, and had no real hand in the new War until we were bombed. Not to mention, many made the same accusations about war-time profiteering by banks and military manufacturers that liberals made during the Iraq and Afghanistan conflicts. However, isolationism during this time was a bipartisan movement.