I share your appreciation for FDR; he’s right up there with Lincoln. Several presidents have inherited colossal messes; great ones rise to the occasion.
Lincoln’s greatness rests on rallying the North to fight secession, initially just to defend the Union and the Constitution, and then seizing opportunities to steer the country toward emancipation en route to ultimate victory. The other men who credibly could have been president at that time — Henry Seward, Stephen Douglas, George McClellan — probably lacked the skill and/or inclination to do that.
As I wrote here, Buchanan did not merely inherit slavery; he actively promoted its preservation and extension throughout a long political career.
Washington would not rate as a great if we judged past presidents solely on slavery or race. He got Americans to grow up and pay taxes to support a functional & solvent central government, and stepped down before he died to show we would have no presidents for life.
To date, Trump has achieved little more than embarrassing this country on a daily basis with brash ignorance. Historians tend to rate on performance rather than cosmetics (exceptions: continued overrating of JFK & JQ Adams).
Trump has not yet caused any irreversible harm. We resist to shorten his term and limit the damage he can do. He will go down in history as a poor president who achieved little. He can only qualify for worst ever if the resistance fails and he kills millions or does something comparably bad.