I think this is a mistake. It doesn't matter what Trump promised, he has demonstrated his base supports him no matter what. On the other hand, wary republicans have been brought into the fold because he's about to give them their heart's desire (tax cuts, regulations cut, lower corporate taxes, re-patriation, etc etc). His base and the GOP base are set. The only way that changes is if he violates something they hold sacred - amnesty, abortion, guns, etc. He's not going to do that so...
In some ways you have to view him like a republican Obama. He has a base that is with him and will not abandon him because "he's on their side." Even mis-steps will be brushed off or ignored as long as he doesn't go too far. Achievement over the next four years doesn't matter as much as having "our man in the White House."
Barring a recession or trade war the economy will be good over the next few years. Americans have already demonstrated they don't give a shit about the rights of various minority groups so I don't really buy the idea that banning trans women from using female bathrooms is going to spark anything, for instance. IF Trump is going to lose it's going to happen because democrats have a candidate who turns out the base in force, and said candidate has the potential to win over fence sitters and others who regret their Trump vote.
We just watched republicans make the same mistake twice against Obama and lose big (08, 12). Let's not dust off their playbook for 2020 and expect Trump's base to abandon him based on our own biases and narratives ("he's a failure, why can't they see it!").