Impeachment is the control against a president acting like a dictator.
But I don't know the control against collusion with congress to empower the president.
It takes 34 out of 100 senators to acquit any impeachment.
So you only need 34 senators to stay blindly loyal to a president and then a president can break any law he wants or do anything at all that he wants.
Regardless of public opinion, the DOJ, The Supreme Court, any elections, etc.
Also, I don't believe the initial House vote for impeachment can happen without the Speaker calling for a vote.
Theoretically, Trump could threaten Paul Ryan's life if he calls for a vote and there would be no ramifications for Trump.
Remember how McConnell would not hold a vote for Obama's nomination of Merrick Garland to the Supreme Court.
And there were no ramifications in place for that.
I believe Trump will push every legal and ethical path to its limit.
And his followers will support every move.