Not sure you are following the logic. It’s not a whataboutism. It is first of all about having a common standard of judgement. If it’s ok for Biden to threaten an aid cut-off in order to force removal of a corrupt prosecutor within six hours, then why isn’t it ok for Trump to request an investigation into Ukrainian interference in the 2016 election?
The other point is that Trump’s suggestion that Biden be investigated is not illegitimate or unjustified. You are sure Joe Biden acted out of pure motives, but the Obama State Department understood that Biden had a perceived conflict of interest. His son Hunter had been paid millions by Burisma. Trump suggested the Ukrainian President investigate possible corruption. The conflict of interest is more than enough to justify his suggestion.