1) I don't think it'll be "easy." I think he could definitely lose in 2020. Especially if the Democrats run a relatable candidate who is a bad matchup for Trump.
2) I agree, a lot of non-Trumpers voted Trump because of Hillary and because they feel their hand is being forced by the powers that be. There is a disconnect. That might be media apologizing for ANTIFA setting cop cars on fire, or a political environment that pretends the women's marches are necessitated by some Handmaid's Tale world Trump is creating or something similarly bizarre. What the middle 50% of the country feels and what the 25% on the left tell them they should feel just aren't that well aligned. I think the same can probably be said for the 25% on the right, to some extent.
3) Trump, his clownish antics and all, is talented in getting the left to move further to the left than is politically useful to them. Kneejerk reactions favoring sanctuary cities and seemingly condoning illegal immigration probably isn't helpful. Coming out and trashing bonuses from companies after the tax break probably isn't a wise move. Trump is for the anthem, so the left is for kneeling during it, etc.
4) I think the right stands to lose ground in the midterms, as you infer. But that may or may not mean anything for 2020.