Trump isn't stupid, and he isn't naïve. I'm sure he knew exactly what he was getting into. For the first time in his adult life,
he was walking into a situation where HE was not the boss. Remember that the Trump Organization is a privately-owned company with no external oversight. As the President of the United States, he would now be answering to a "board of directors" (Congress) for the first time.
The reason I'm confident of all this is that the Trump administration has gone to great lengths to play everything according to the rule of law. You saw clear evidence of this in Barr's work to bring the Mueller investigation to a close (i.e., he has done EXACTLY what the law and U.S. Justice Dept. regulations require). For all his flaws, Jeff Sessions operated the same way.
I think he probably expected the Democrats to be back-stabbing liars; and he probably expected a few of the GOP people to be back-stabbing liars. But, I'm not sure he had any idea of how many of the career civil servants were conniving, partisan hacks. Especially in the FBI and intelligence community.
He essentially arrived in Washington without any friends there.
At least in the business world, you have a pretty good idea where people stand. They may not always be honest, but they adhere to similar customs and traditions.