I'm afraid some conservatives are being misled by Judge Luttig.
Of course, you can impeach, convict, and disqualify a former officeholder.
This view is supported by English custom, Constitutional text and structure, original understanding, and continuous Senate precedent.

When the Founders were debating impeachment, they had in mind the then contemporaneous impeachment of the FORMER Governor-General of Bengal, Warren Hastings. That impeachment was led by the INTELLECTUAL FOUNDER OF CONSERVATIVISM Edmund Burke.

The impeachment power includes the power to disqualify for life. If officeholders could simply resign to evade this important punishment, it would render that a nullity.

If you read the debates on the Constitution and public commentary of the founding generation, it's absolutely clear that *at least* half of the purpose of impeachment is to inflict public punishment on political "crimes" against the Constitution, not simply removal from office.

The Senate has repeatedly affirmed it's power to try former officeholders.

This is not a close question.