"Only?" I don't think that is an acceptable litmus test. If something is 99% corrupt and 1% in the "interest" of the nation, that's impeachable wrongdoing IMO. Further, the "where is the evidence Trump did not do this in the best interest of the nation," can easily be turned around and asked of Joe Biden's situation. The difference being that Biden has numerous people, entities, and nations that would back his actions as being appropriate. The president doesn't have that. What he does have is administration officials, decorated soldiers, ambassadors, all indicating they had issue with his actions and silence by the Ukrainian govt not want to get involved.... Wait, I'll amend that. The president does have someone that says he's doing the right thing.... The guy currently involved in a hot war with the Ukraine and whose intelligence agencies assisted in the election of the president; ole Vlad.
And investigating the Biden's? What proof do people have that there was any wrongdoing? Now, if you hold that the investigation into Trump, was witchhunt, hoax, or illegitimate... don't you have to say the same for the Biden's given your statement?
You are certainly entitled to your opinion, but I too would like to see the where in law or statutes that a sitting President asking a foreign government to investigate anyone suspected of the type of wrongdoing listed below is even unethical.
As for your final point, if Don Jr. was pulling millions out of a country DJT was bragging he threatened to withhold aid from and had a prosector fired in you would be going nuts on it. That you fail to acknowledge that major difference is enlightening...