Neither the White House Press Secretary nor the White House Chief of Staff are subject to Senate approval, so I guess if Trump decides that he doesn't like Stephen Miller as Chief of Staff or Dana White as Press Secretary (assuming that he does take the position), even after the Democrats take control of the Senate January 3, he can still fire and replace them.

He's going to be a lot more locked-in when it comes to positions subject to Senate approval. He can still fire them, but he probably won't get replacements that he likes more past a Democratic Senate, so that'll decrease his influence over his Cabinet members.

So if he's going to kick any of those people out, he probably wants to do so in the next seven-and-a-half weeks and get their replacement in. After that window closes, he's kinda stuck with them for the remainder of his term.