An analogous situation to Hunter being on the board of Burisma would be the people on the board of Theranos. If you remember, Theranos was creating a new way of doing blood tests and they raised hundreds of millions of dollars. However, their technology didn't work and they hid the failures from everyone. The board had a lot of top military and political people, such as Henry Kissinger, James Mattis, and George Shultz among others. None of the people like that had any experience in developing medical testing equipment. They were likely hired for the board for other reasons, like their contacts, their advice on business, their ability to organize and execute, etc.
Trump holding back aid to Ukraine until they investigated Biden+Burisma would be like S. Korea holding back military compensation payments to the US until they investigated Mattis+Theranos. Even if those companies are corrupt, they are essentially unrelated to the money being held back. And just because a big name is on the board doesn't mean they got there because of corrupt reasons.
You clearly don't see the difference.
Which of those people you named who were on the Theranos board had a close relative in a position of power within a governmental agency which had some kind of authority over Theranos? Were those board members making millions of dollars being on that board, and did they have a close relative making sure any investigation was ended by having the investigator fired? And did their relative have the power to steer billions of dollars to Theranos?
Or, were those people simply high profile and probably with important connections to open the right doors?
Your example of S Korea holding back funds from the USA is laughably upside down from the USA giving billions to Ukraine. The scale of dollars between the 2 is as different as could be unless S Korea were sending us trillions of dollars.
Even so, ignoring that, what interest does S Korea have in any of those people or Theranos? Are the alleging violations of S Korean law by those people? If so, they have a legitimate reason to ask for US help in the investigation.
Furthermore, the US aid to Ukraine was not contractual. It was a gift. A very large gift. Under the Obama administration there were billions given which basically disappeared into the corrupt black hole, aka lined the vaults of whichever corrupt Ukrainians were able to snag it. Under Trump, the foreign aid was a gift which we have the right to put conditions on. That is how foreign relations work. It is always give something to get something.