The first day of office he should have purged every single people that were not completely loyal to him from every single US administration that depends on the executive branch and replace them with his people. He didn't even do that with the white house.

You can thank Russiagate for preventing this, or if you want to go one step further, Mitch McConnell and the GOP establishment.

I'm sure that at some early point Trump realized he needed to fire the entire bureaucracy, but also realized that Republicans would help Dems impeach him Caesar-style if he ever moved to do so. As DistilledLife says, that might have been a good thing, but we also might have American troops dying overseas if not for Trump's term as well.

He should have pardoned Cohen and co in advance to build loyalty, he didn't. Now nobody is going to be loyal to Trump.

I somewhat agree with this. However, he couldn't have pardoned Jenna Ellis or anyone else who hadn't even been charged when he left office.