1. “Abuse of Power is an opinion.” Yes. But It’s an opinion that Congress gets to have. This is provided for in our Constitution, which says that impeachment may be pursued for treason, bribery, and “other high crimes and misdemeanors.” The Founders debated heavily about that last clause, and ultimately kept it with the intent of making impeachment somewhat flexible. Most scholars agree that impeachment-worthy actions are defined by Congress. When it comes to the POTUS, we usually are not talking about routine crimes like murder that have a prior definition. The President by virtue of his office is capable of undertaking a wide range of possible troubling actions that may have never been seen before.
Your observation that Republicans didn’t impeach Obama is pretty empty, considering that Republicans did impeach the Democratic president before him for lying about a BJ. Perjury is a crime, yes, and he deserved to lose his law license. But it is not exactly a “high crime or misdemeanor” in the judgment of many. Congress was still allowed to proceed on that theory, because Congress gets to define it.
What Trump has been credibly accused of re: Ukraine is not as tabloid-ready as a BJ, but is far worse from a democratic standpoint: using the weight of the office of President to pressure a foreign power to open an investigation on the son of a political opponent. Because of the foreign policy angle, even if there wasn’t a literal burglary, this is worse than Watergate, IMO: and there is smoking-gun evidence in the Zelensky transcript that he sought to do this.
2. There are complex arguments about the scope of Executive privilege, but Congress certainly gets to have a say about presidents defying its subpoenas, up to and including impeachment. There is certainly nothing in the Constitution that says Congress need to wait for a review by SCOTUS before exercising its impeachment powers.
No, our constitutional norms are not the problem. Trump’s inability to comply with them is.