I disagree, if Trump wins and wins big he will have a mandidate from the people to advance his agenda and his agenda will advance. But for the sake of argument lets say you are correct.
Trump does not need Congresssional approval to enforce existing immigration law. As President he has all the authority he needs to deport illegals. The wall has already been approved by Congress so if Trump can figured out a way to fund it he can build it without Congress. As President he can halt immigration to the USA by Muslims without Congress. He can send refugees home without Congress. And the can renegotiate trade deals without Congress. So much of Trumps agenda does not rely on Congressional approval.
But as I said if Trump wins large (and he will) then Congress will go along with his agenda and pass his legislation he is the consummate deal maker, the master of negotiating to get his way remember?
Excellent point on immigration. All he has to do is enforce existing law. Nothing radical about that.