As for the CIA's effectiveness in the latter stages of the Cold War, in time, internal intelligence documents and studies get released that allow a qualified assessment. As it happens, under Bill Casey and his successors under Reagan and Bush, the CIA did a fairly good job of helping bring down the USSR.
Most notably, the CIA provided arms to the Afghan resistance and cash and other resources to sustain the anticommunist Solidarity movement in Poland. The CIA also recruited a highly placed Polish military officer, Ryszard Kukliński, whose reporting from 1972 to 1981 yielded 35,000 pages of mostly Soviet secret documents. This provided the US and NATO with an edge in assessing and countering Soviet military readiness and capabilities.
According to Carter's National Security Adviser, Zbigniew Brzeziński, "Kukliński's information permitted us to make counterplans to disrupt command-and-control facilities rather than only relying on a massive counterattack on forward positions, which would have hit Poland."
Like some of the best spies, Kuklinski had a good record and acted from patriotic and moral principles. He and his wife and two sons were exfiltrated by the CIA in late 1981. Kukliński died in a hospital in Tampa in 2004. His remains were eventually repatriated to Poland and reinterred with honors in a military cemetery.
There is a great deal that we do not know, but Congressional oversight seems mostly satisfied that the US gets sufficient value for the cost to maintain the CIA and other spy agencies.
Some oversight, when you had the CIA, spying on the U.S. Senate during Obama's administration, and of course, no one was ever held accountable for it, and it's likely that the CIA and other agencies have never stopped spying on Congress. They've been abusing their powers for eons, and will continue with impunity. I'm not gullible enough to believe that Congressional Oversight knows what any of them are truly up to. Congress only knows, what the CIA allows them to know. And Congress, in reality, doesn't really want to know, because then they might have to do something about it, and neither party is worthy of anyone's trust these days, to do the right thing.