Did we use any of the reserve fleet submarines after WW2?

We did. We reactivated and recommissioned a number of these boats for service during the Korean Conflict in the early 1950s. A few of those were kept active after the fighting ended but most went back into the reserve fleet. 

Other boats were sold to allies and friendly nations. Some of those were given upgrades first. Other boats from the reserve fleet were used as targets when testing new torpedoes. Still others were used in the atomic bomb testing in the Pacific. Lastly, some of the WW2 boats became museum submarines, such as the USS Pampanito (SS-383) here in San Francisco.

By the late 1950s, we were building nuclear powered submarines. That meant that the WW2 boats were quickly outdated. By the end of 1973 nearly all of the World War 2 submarines had been decommissioned. However, some of the new classes of diesel submarines that we had built after the war were still in service in the U. S. Navy as late as 1988.

Two of our WW2 submarines, the USS Tusk (SS-426) and the USS Cutlass (SS-478) were transferred to Taiwan and are still in service as of 2025. However, since they are now 80 years old, they are limited in their capabilities. Taiwan is now building new, non-nuclear submarines.

NOTE: Some of the battleships that were in the U. S. reserve fleet were returned to service as late as the First Gulf War. They had been upgraded to carry and launch tomahawk missiles. All of the Iowa class battleships, the last class, are now museums.