Gruenberg should resign immediately and Democrats should prioritize doing the right thing over politics and joining Republicans in calling for him to step down. Democrats have called for numerous officials to be fired or to resign over similar improprieties, and Gruenberg's status as a Democrat shouldn't protect him.
In an ideal world, Gruenberg would be replaced, but political logistics make the move risky. Regardless of how quickly the Senate acts, it could be months to name his replacement. This could leave the FDIC in a weak position to promote key reforms during a time of banking uncertainty.
There's a 50% chance that at least 138 FDIC banks will fail in the time period from 2023 through 2030, according to the Metaculus prediction community.