Presidents have immunity from criminal prosecution for official or unofficial acts during their presidency that they have not been impeached and convicted for. In this case, it's clear to even the folks bringing the charges, that Trump has done nothing illegal. And when there is a Senate trial that has resulted in acquittal, there can be no charges later.
without a cite to a precedent or a law you are just wishing it to be true.. A senate trial is not relevant....
Swing and a miss...
Uunder your scenerio, a president can rape an intern and simply resign and declare immunity.
Also how do you have immunity for an unofficial act you have not been convicted for.. that is like saying you have immunity for any crimes you did not commit....
Strike 2