Merchan donated to Biden and democrat causes....

As is allowed, consistent with judicial ethics. Judges are people. They have political beliefs. Almost every one of them probably voted in the last Presidential election. They sometimes contribute to political campaigns. Some will have voted for Biden, some for Trump. All of that is normal and permissible in the legal system. It's not unfair that your judge will have a predilection for one of the two major political parties in the U.S. It is not unfair to the prosecution to have pulled Cannon (also at random), and it is not unfair for the defense to have pulled Merchan.

Sure, when you hold a trial in a district that hates a guy, you're guaranteed to get a jury pool that knows what to do.

No, you're not. Unless you're willing to give up on the idea that the criminal justice as a whole could ever be administered fairly, you have to postulate that a random selection of people processed through the mutual jury selection powers that both the prosecution and defense have is capable of returning a fair verdict.

Are you willing to open up all the jails and prisons because no one got a fair trial?