I’m not convinced this is necessary. The President has the power to pardon whomever he/she wishes, but no one is above the law. Accepting a presidential pardon is an admission of guilt, and, once pardoned, that person can no longer claim a fifth amendment protection when called/subpoenaed to testify. Just because a pardon is in place does not mean Congress cannot access the records they need to pursue an investigation. There is an office to screen the pardons the President wants to make - Trump, in his arrogance, chooses not to use it.