You are assuming away the main issue, ie exactly what law did the President break? If, for example, he had demanded they supply him with underage girls to get the money (not mentioning any names here), then I agree there is a big problem. But this is completely different from the president using aid as leverage to get action from another country on a matter of legitimate national interest.
That remains to be seen but it did not seem relevant to Trump or GOP for 2 years until Biden was the front runner so it was not a matter of national interest, only to Trump.
I dont think the impeachment is going anywhere but your stating that it is of national interest if Biden pressured Ukraine to fire the DA before getting any aid. If that is true then it can be brought up repeatedly by GOP during the election to discredit Biden.
But to go to another country and ask them to investigate something Biden did and threaten to withhold aid, if true, is presently the issue. How do you claim what Biden did is wrong but Trump was ok since both really were self serving. Bringing Biden does not defend Trump it makes what he did look just as bad and he of the two is in office.