I'm not a lawyer, and I haven't followed the details, but I think, in some way, Nancy Pelosi is responsible for Trump having the winning argument.

Constitutionally, the only remedy for a sitting President is impeachment by the House and conviction by the Senate followed by removal from office. After that, presumably the President is subject to further litigation over that same issue.

Prior to 2021, one would think that the impeachment clause only covers sitting Presidents. But thanks to Nancy impeaching again a week or so before the end of his term, Trump was tried in the Senate after his term had expired.

That Congress didn't abandon the proceeding would indicate that they inferred that the impeachment clause still applied to the President even after being out of office. I think you can make the legal argument that Congress continuing the impeachment process after the term expired means that trial of a former President on any issue would be subject first to impeachment in the House and then conviction in the Senate - otherwise why continue after Trump left office?

Since he wasn't convicted in the Senate, then he can't be tried for actions while in office.