Impeachment has nothing to do with criminal prosecution. As the constitution itself specifies:no where to go
the Dems had their shot to try trump constitutionally through impeachment. Instead they blew their wad immediately. Without doing any investigation. Trump as president was tried via the mechanism of the constitution for criminal activity by a president in office. That is impeachment and he was acquitted. Yes anythibg that he is accused of doing while in office must be conducted through impeachment. Theoretically he can be impeached at anytime even after leaving office.
Article 1, Section 3: "Judgment in Cases of Impeachment shall not extend further than to removal from Office, and disqualification to hold and enjoy any Office of honor, Trust or Profit under the United States: but the Party convicted shall nevertheless be liable and subject to Indictment, Trial, Judgment and Punishment, according to Law."
It says the opposite of what you're claiming. It specifies impeachment can carry no additional penalty beyond removal and disqualification, and that all further penalties are handled by criminal prosecution, which the impeached is still subject to.