There has to be: 1) Immediate emergency hearings in the House. 2) Lawsuits to end the sabotage and reinstate the sorting machines, overtime, etc. 3) State level investigations, indictments, and civil suits.
While I agree as you point out the only powerful oversight mechanism over the USPS have been compromised by Trump and an "ally". So I think we have to call the USPS dead for now and focus on drop off and in person voting plans around the nation. They already won the USPS battle.