The president is immune from prosecution during the tenure of his presidency. Impeachment of a sitting president is the means whereby he may be removed from office. Once the president is no longer in office impeachment is moot. Using impeachment of an ex president to preclude him from holding office again is extra constitutional.
Totally agree. Impeachment serves as a check on a sitting president by a co-equal branch of government, the legislative branch. Trump is now a private citizen and therefore, as you note, this whole charade becomes moot.