To any trumper still clinging to the "but impeachment of an ex-potus is unconstitutional" line - this puts the nail in the coffin.
"Whether a former officer can be impeached is beside the point. Donald Trump was President of the United States at the time he was impeached by the House of Representatives. The impeachment was therefore unquestionably permissible (putting aside any disagreements over the nature of the charges).
Article I, Section 3, Clause 6, states: "The Senate shall have the sole Power to try all Impeachments." The key word is "all." This clause contains no reservation or limitation. It does not say "the Senate has power to try impeachments against sitting officers." Given that the impeachment of Mr. Trump was legitimate, the text makes clear that the Senate has power to try that impeachment.
Article II, Section 4, states: "The President, Vice President and all civil Officers of the United States, shall be removed from Office on Impeachment for, and Conviction of, Treason, Bribery, or other high Crimes and Misdemeanors." This provision does appear applicable only to sitting officers. But it does not limit the power of the Senate to try, which comes from Article I, Section 3, Clause 6. It merely states that removal from office is mandatory upon conviction of any sitting officer. No lesser sanction will suffice.
Article I, Section 3, Clause 7, states: "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." Read together with Article II, Section 4, this means that the consequence of conviction on impeachment must include removal from office, may include disqualification from future office, and may not include any other sanction. The first sanction is limited to sitting officers, which makes sense. The second sanction is not so limited.
Some argue that the conjunction "and" in Article I, Section 3, Clause 7, implies that the sanction must include both removal and disqualification, and that because removal of a former officer is not possible, disqualification must also not be allowed. But the clause does not say that both sanctions are required; it says that the judgment may not go beyond imposition of both sanctions."
https://reason.com/volokh/2021/01/28/impeaching-officials-while-theyre-in-office-but-trying-them-after-they-leave/