I don't think Trump will ever quit, even if he does lose in 2020. He has a good chance of winning reelection because the incumbent has the advantage. I think he'll likely lose the Senate in 2020, there are too many GOP senators up for reelection. Once the Senate goes blue, it will be much easier to get the 2/3 vote. Pence will become the 46th President in 2022.
There is a small chance that the Dems will get the WH and the Congress. At that point, the country will become a socialist hell hole and will be time to drop all our guns into the ocean, allegedly.
Any half decent Republican can do everything that Trump has done, reduce immigration, lower taxes, impose tarrifs (which is a form of tax), promote the pro-life agenda, fix the Middle East one way or the other, fire striking Union workers (wait, that was Reagan). President Pence can do all of these. Just get ready for a string of purity executive orders...