If you empanel a grand jury and present them with compelling evidence that the president accepted a bribe for a pardon, you could presumably indict them.

From there, you would present this evidence that there was a quid-pro-quo bribe and presumably the defense would move to dismiss under "it was an official act, can't prosecute". The judge would then need to decide if there is sufficient evidence to call into question if the act was official, given that the president cannot give an illegal order as an official act. If there's enough evidence, presumably the judge wouldn't dismiss and the trial would continue. (If they did dismiss, presumably the prosecution could appeal to a higher level court,)

I am just not clear on why everyone both thinks, and seem to want to think that this has given up the ball game and now the president is a king.

I am trying to argue in good faith. I just don't agree with you that the president can now do whatever they want. If they could, Biden could order the assassinations of all Republicans sitting in congress, for instance - presuming your reading of this is correct, what's to stop him? If you think it's just that he's not bold enough, perhaps you should call the whitehouse and give your opinion on what he should do with his newfound powers.

And from the dissents:

Whether described as presumptive or absolute, under the majority’s rule, a President’s use of any official power for any purpose, even the most corrupt, is immune from prosecution. That is just as bad as it sounds, and it is baseless. Finally, the majority declares that evidence concerning acts for which the President is immune can play no role in any criminal prosecution against him. ... Whenever the President wields the enormous power of his office, the majority says, the criminal law (at least presumptively) cannot touch him. ... It says that whenever the President acts in a way that is “ ‘not manifestly or palpably beyond [his] authority,’ ” he is taking official action. ... Under that rule, any use of official power for any purpose, even the most corrupt purpose indicated by objective evidence of the most corrupt motives and intent, remains official and immune. Under the majority’s test, if it can be called a test, the category of Presidential action that can be deemed “unofficial” is destined to be vanishingly small. ... Under that view of core powers, even fabricating evidence and insisting the Department use it in a criminal case could be covered. ... Imagine a President states in an official speech that he intends to stop a political rival from passing legislation that he opposes, no matter what it takes to do so (official act). He then hires a private hitman to murder that political rival (unofficial act). Under the majority’s rule, the murder indictment could include no allegation of the President’s public admission of premeditated intent to support the mens rea of murder. ... The President of the United States is the most powerful person in the country, and possibly the world. When he uses his official powers in any way, under the majority’s reasoning, he now will be insulated from criminal prosecution. ... Thus, even a hypothetical President who admits to having ordered the assassinations of his political rivals or critics, or one who indisputably instigates an unsuccessful coup, has a fair shot at getting immunity under the majority’s new Presidential accountability model. ... Also, under the new Presidential accountability model, the starting presumption is that the criminal law does not apply to Presidents, no matter how obviously illegal, harmful, or unacceptable a President’s official behavior might be. ... Unlike a defendant who invokes an affirmative defense and relies on a legal determination that there was a good reason for his otherwise unlawful conduct, a former President invoking immunity relies on the premise that he can do whatever he wants, however he wants, so long as he uses his “ ‘official power’ ” in doing so. ... From this day forward, Presidents of tomorrow will be free to exercise the Commander-in-Chief powers, the foreign-affairs powers, and all the vast law enforcement powers enshrined in Article II however they please—including in ways that Congress has deemed criminal and that have potentially grave consequences for the rights and liberties of Americans.