Trump would resign before allowing himself to be impeached, if it came to that. As it is, I think that the Republicans look at him as a useful, if somewhat repugnant distraction from their dirty deeds.
I honestly don't think he'll run a second term. I noticed and thought at the time, and nearly everything out of his mouth and from his fingers since has convinced me that whatever he accuses his perceived opponents of being/doing, he, himself is guilty of, several times over. During the elections, not only did Trump accuse Clinton of being crooked but also stated over and over that she didn't have the stamina to serve.
Says the man child who is perpetually on vacation or else on the campaign trail--for himself.
FWIW, I think that Pence would be as bad, in a different way, than Trump. Ryan? No.