Truman is now seen as a near-great president because he put in place the containment doctrine boosted by the Truman Doctrine and the Marshall Plan and NATO, which historians now see as having been at the center of American success in the cold war.
Harry Truman wrote scathing letters, but he almost never sent them.
At the end of their first years, there are few people who would have predicted that Truman would be elected in 1948 or that Reagan would get a second term. It’s always premature to make some kind of categorical judgment after the first year in office.
There are examples of ex-presidents speaking out. Jimmy Carter has not held back on a variety of issues. Harry Truman didn’t.