Trump has shown us what he is, a loudmouth who's biggest boasts are empty. He doesn't mean what he says or signal serious intent; his words are meant for no more than their momentary effect. Every line is intended as an applause line, so he can bask in the glow of followers' adulation.
While Trump is a man who would sacrifice anyone to get what he wants if it was easy to do so, he is also a man who quits and shifts focus when he hits a road block, and who doesn't (fortunately) have a determined enough hatred of any group to target them at any cost. He is not a Hitler; he is just into self-aggrandizement.
Yes, he'll do some harm if re-elected, and a sociopathic narcissist should never be allowed the reins of power. But your worst fears are wildly implausible. Trump wants the cheap applause, not to become known as the man who destroyed American democracy.