I agree Assange should not have been pursued at all.
You can't directly compare the two situations. Manning was a US citizen, serving a sentence in the US. During Obama's term, Assange was not yet accused of any crime in the US, so there was no presidential pardon to be had, though Obama could have dropped the extradition request.
Biden did eventually reach a deal with Assange that allowed him to count time served -- essentially the same as commuting a sentence, so along the lines of what Manning got -- and return to Australia.
So in the end, Assange and Manning were both freed after serving time.
Yes he could have. And did not. Whereas Manning was freed, because s/he had sponsors in the right lobbies.
> Biden did eventually reach a deal with Assange
Mostly driven by the UK government, because it would have looked bad to deport a journalist on the orders of our colonial masters. And again it took years, when it could have been done in a minute - like Trump did with his allies. Because the reality, beyond words, is that the Democratic establishment is basically as much an enemy of the free press as the Republican one is, when in power.