When Trump was impeached, the big issue was that he wanted to get dirt on his political opponent. He wanted dirt on Joe Biden's son and how he worked with an oil company and wanted to accuse Biden of nepotism.
That's really not why Trump got impeached.
First of all, everybody collects dirt on their political rivals - nothing illegal about that. If Trump wanted to hire someone to dig up dirt on Joe, that's not an issue. Alternative if Trump wanted to "serve justice" on Hunter and Joe Biden for corruption, President Trump could have had the US Justice Department investigate and prosecute them on whatever evidence they could find. In such a scenario, the Justice Dept. could have enlisted the State Dept. for cooperation from Ukraine.
Trump got impeached because he tried to engineer a foreign investigation of the Bidens, one devoid of his own fingerprints - and more specifically, for using the powers of the Presidency to coerce Ukraine into the investigation. The Democrats impeached Trump for "abuse of power", saying that coercion of Ukraine by the President in this way primarily benefited Trump, not the United States. Using the powers of your office for personal benefit is a definition of "abuse of power". Impeachment was cited by the founding fathers as a remedy for several specific offenses; abuse of power was one cited by Alexander Hamilton.
Secondly, Trump was impeached for obstruction of Congress. According to one commentator:
The White House refused to provide documents to congressional
investigators, and instructed top advisers and government officials to
defy subpoenas and refuse to testify.
This was a step that not even Richard Nixon tried.
what was the point of trying to get dirt on Hunter Biden if the Mueller report, for example, didn't change enough minds to mean anything?
Having a foreign government condemn Joe Biden for corruption could have been portrayed as a politically unbiased indictment of the Bidens, thus being politically more damaging. If the timing of such a disclosure could have been controlled, it could have cast a huge shadow over the 2020 election, and allowed President Trump an easier path to reelection by making his perspective rival unelectable in many voters' eyes.