… as long as the Democrats in the Senate don’t convict him in an impeachment trial.
That’s obviously false, for many reasons. If it were true, any President could commit any crime he wanted to commit, even with a hostile Senate and Vice-President. All he would need to do is resign before being impeached, and never face any consequences. For example, Biden could have Trump assassinated, then resign before being impeached, allowing him to spend his remaining years basking at the poolside in his Aviators, telling Cornpop stories.
It’s difficult to believe that any court would accept such an argument, but who knows these days?
Of course, Trump’s lawyers don’t really think this is a valid argument, and neither does he.
Remember his argument that the vice-president could reject state slates of electors? Well, he doesn’t really believe that one either. He doesn’t think that Kamala Harris can throw out the electors of any state that votes against Biden in the next election.
In both cases it is/was simply expedient to hold those positions temporarily.
The strategy in this case is to file time-wasting motions in order to keep delaying his federal trials until after the election, with the hope that he will win the vote, and thus control the DoJ. When the immunity maneuver is rejected, he’ll come up with more nonsensical, time-killing arguments, probably at the very last minute of the period allowed, and hope they move slowly through the courts.