So, would official acts as president be legal by definition?

yes, and further that any exercise of constitutional authority is an official act.

Would there be such a thing as an official act as president that may otherwise be criminal?

in the prosecutable sense? no. the president is no longer bound by congressional authority.

And how does the ruling protect against treason, bribery, or other high crimes and misdemeanors (specifically, the past part)?

courts won't do shit about it, congress will have to (lmao)

How is this ruling not in direct contrast to the constitution?

the constitution is toilet paper and always has been. scotus just wiped some diarrhea with it.