I'm dismayed by this ruling but I'm curious: can someone defend it? I'm able to understand the counter-perspectives to my own on many hot-button issues (2nd amendment, abortion bans) but this one seems very nakedly bad. But maybe I'm just not seeing the counterpoint?
Sure. You want a President to be able to carry out the roles of the office without concern that his or her political opponents will use the courts to try to punish those actions. There are reasonable disagreements on where Presidential authority begins or ends on many topics, and you want the limits to be either through separation of powers (e.g., the Judicial Branch can bring an end to actions, the Legislative can impeach and remove the President) or through the ballot box.
This does not mean, including from the majority opinion, that anything the President does is immune from challenges. If the President directs a cover-up for his campaign (Nixon) or directs a Governor to find enough ballots for him to win or directs "alternate electors" via fraud (Trump), this is not an official action. Trump's lawyers admitted as much in the oral arguments.