It's not illegal when the president does it. So the Supreme Court has said.

No, it didn't. Go reread the decision. It states that the President has absolute immunity for any enumerated power. That's every power that the Constitution explicitly gives to the Executive branch instead of one of the others. If the Constitution says that the President can, or indeed must, do a thing then no court may rule it a violation of the law.

I think you're missing a nuance. When Trump is the president, it's not illegal when the president does it.

For now, true. We can't generalize until there are more presidents with this supreme power.

I kind of want to see him take gold from Fort Knox for some "official purpose". Why not? It'd be such a flex, if we're getting this treatment it may as well be interesting.