I don't expect Trump to run in 2020 after being impeached by the House in 2019 or early 2020. after a long set of hearings.
Given that the Democratic Party will likely give the nomination to someone from the far left, I expect that Rubio, Haily, Cruz or Kasich would have no trouble winning.
Against Trump, I suspect that Biden would be the strongest candidate.
Those impeachment dates just get pushed further and further back.
But an impeachment in 2020 is extremely unlikely, if politicians even have a modicum of sense and self-preservation instincts. The only way it could possibly work is if President Trump did something not only ridiculously illegal, but also highly unpopular across the board.
If he gets impeached merely for being unpopular among the left or uncouth or due to unproven conspiracy theories or for enforcing immigration law or for whatever reason, and this happens at the start of the 2020 election season, Americans will interpret this as a clear signal that President Trump is being impeached merely because the political elite have decided that he is not acceptable in their society, regardless of what the voting public thinks.
The biggest reason that President Trump won the election was the feeling among Americans that they had been ignored by the establishment, and that if they elected a usual politician their concerns would continue to be ignored. If he had run 20 years ago, he wouldn't have won, because people would have had too much faith in politics as usual.
You may be hoping that if President Trump is impeached at the beginning of the election season, then the voters will resign themselves to the idea that they have to accept the poor candidates that the establishment parties give them, regardless of how little they do for them. But that's not what would happen. Rather, the public would learn that they won't be represented by the president, or any other politician for that matter, and as such they will have to make their will known in non-political ways. If you don't like President Trump you
really won't like what that looks like.