He will be in a federal prison before then
That's not going to happen. He's more likely to serve a full term no matter how bad the Mueller investigation gets for him. The only way Republicans turn on him is if they lose the House AND Senate tomorrow. If they keep the Senate, they will take that as a sign that Trumpism gives them a chance to win re-election in 2020. If they were to remove Trump from office, it's guaranteed Pence loses in 2020. Plus they would probably believe Trump impeachment from the House, but not a removal from the Senate would rally up his base for the 2020 election because they would believe he's under attack.
If the Mueller investigation concludes in 2019 and its as bad as we think, expect McConnell to say something like
"Its close to an election year so we think the American people should decide Trump's fate" or
"We don't think the President's actions rise to removal from office but it's very concerning."
Or something dumb like that. Republicans only care about power. I use to think, like you that he wouldn't make it a full term, but Republican behavior says otherwise. I'd like to be wrong though.