I am not a lawyer, and I do not know how the law would apply in this situation, if at all. However, it seems that since Trump is basically acting for his own benefit or out of personal spite, which he has openly admitted on several occasions, rather than acting with equal regard on behalf of the citizenry, that he can be sued as a private citizen who is purposefully causing harm with his disregard of proper pandemic procedures or his interference in the CDC and the USPS, among other things, and that he should be liable for damages from his own personal funds and not the funds of the government because his actions are for his own emotional or personal benefit and not the benefit of the citizenry. I don’t see how he could claim that he has immunity from such a suit by virtue of being president when he otherwise disregards the Constitution.
For similar reasons Dejoy could be sued from his personal funds for the damages he is causing people who, for example, are not receiving medicine or important papers in a timely fashion, including things like ballots. People who have been affected could join a class action suit against Trump or Dejoy and threaten the only thing they care about, which is their own money.
If millions of people joined such class-action suits against Trump and Dejoy, these men could be brought into civil court and forced to defend their actions in a court of law. It seems it should be possible since their actions are motivated primarily by personal gain. Dejoy has financial interests in USPS’s competitors and Trump is abusing the government to aid in his reelection and punish people he doesn’t like. It seems like a case could be made.