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.