@AndyD He has always demanded fealty from his underlings, and he made the mistake (in his mind) during his first administration of having people in his cabinet who didn't execute his every instruction.
He's a sucker for praise, and the so-called adults he's surrounding himself this time around are getting their respective jobs because they have kowtowed to him. They have few scruples and their own agendas, so they won't do anything to stop him.
I doubt that his party in the legislative branch (which has the majority) will have the guts to speak out either. We saw some of condemn him after January 6th and then cave and support him eventually. If he declared martial law, you won't see legislators fighting to get into the Capitol building to vote against the president as the South Koreans just did after theirs declared martial law.
eta: The Kash Patel Principle, or creating power ministries .
https://www.msn.com/en-us/news/opinion/the-kash-patel-principle/ar-AA1v3mns