The justice dept went easy on Trump because it sets a very dangerous precedent for the current administration to use the power of the justice dept on political rivals. He was removed from office and his actions were investigated and displayed to the public. Under normal circumstances, those actions should make it so he cannot run again. The electorate are designed to be the check on political power, but it failed.
I fear elections no longer have that check. I do however believe the justice department made the right decision. I don't think it should criminally prosecute political rivals, because then we end up with situations like Nivalny dieing in prison. The justice department played it's role by exposing all of the criminal behavior, the electorate did not by allowing someone that dangerous back into power.
I understand your frustration, and I think he is guilty of the things he is accused of also. I still think the justice department made the correct democratic decision of setting the precedent that the executive branch does not prosecute political figures when the electorate has a chance to make that decision.
I hate that the electorate decided that none of those offenses were damning enough to flush that turd, but that's democracy. He won the popular vote and it's up to those of us unhappy with the result to convince others that we need better leadership.