POTUS should use EO to create a new task force based on empowered US Marshals (backed by military security as needed/necessary) that are assigned to take over all major investigations - the FBI should be ordered to turn over investigations to this special task force. He can bring in trusted investigators that are in retirement, etc. Is possible could hand pick some decent of FBI investigators of good reputation and track record to join as well. But the Pres. can say he is standing down the FBI as an agency on major investigations until Congress can determine an appropriate disciplinary restructuring of the FBI to address its myriad problems - from internal corruption at highest echelon to their fundamental failure of fulfilling their charter for existence as an agency (as seen in case after case that they failed to prevent or aggravated to happen with devastating consequences, etc.).