I don't know the legality of Trump's trial after office, but a state-level defendant can be tried even in absentia. Regardless, the impeachment trial or exercise can be one of procedure or Congressional precedent. Otherwise, any U.S. president can wait until the end of his or her tenure or term to commit an impeachable offense.
A comment on "McConnell seeks to push Trump impeachment trial to February" (M.C. Jalonick and L. Mascaro).
Related: "Permanent Record" (2020)
Related: U.S. Space Force (2020)
Related: "Fake" News (2019)
Related: Public Intellectuals (2019)