Impeachment and conviction can impose the additional penalty of disqualification from federal office in addition to removal from office. Whether that can legally be done after a person has left office is an open legal question. In the only case where it was tried, the person was not convicted.
Bingo. Seems that's the threshold question. If the impeachers can't answer it, the whole process is rendered moot, as others astutely have noted.
Trump can only be convicted of committing an actual statute crime, and the only penalty congress can impose is removal from office.
Disqualification from office would require conviction by a federal court after an impeachment conviction.
The congress has no power to enforce federal law.
However see post 62.