According to the constitution, impeachment is literally for treason, bribery, high crimes or other serious misdemeanours. But the SC basically said it fundamentally can not be treason, bribery, a high crime or any misdemeanour if he did something in his role as president. So the whole process has become stale.
Ultimately, the Senate decides on whether to convict/remove for impeachment. The SC does not decide it. Sure, I imagine the Senate would generally want to broadly stay in agreement with the SC, but they don't have any obligation to do so.
At least that is my understanding; I'm not a lawyer or constitutional scholar :)
That's not how it works - in the case of impeachment the Senate holds a trial, and they are allowed to use their own definitions of treason, bribery, etc. The Supreme Court is in charge of what the regular courts do and can make rulings that bind them, but they can't bind Congress in the same way. After all, the justices of the supreme court are also subject to the impeachment process like the President is and it'd be weird if they could make rules about how that works.
You added the adjective "serious" to misdemeanors, which is not in the Constitution. The "high" in "high crimes and misdemeanors" means crimes and misdemeanors committed by high officials, not crimes and misdemeanors that are extreme.
The constitution actually doesn't really say anything about what misdemeanours are applicable in this case. This part has been shaped over time in congress, similar to how common law is shaped by courts.
If I understand you correctly, then if a President's cabinet (high officials) break the law the President can be impeached. Which makes sense to me. It places accountability on the President to pick trust worthy people and to immediately get rid of them if they break the law (to lessen the chances they'd be impeached for not doing something about it and insuring they are prosecuted).
