It is their job to get him in and bring him to the police station. He did not allow them to do that by resisting. Floyd never wanted to harm anyone, he was just high and drunk and afraid of getting to prison again. That's often when it escalates. Something happened in the car that made him bleed out of his mouth (I assume he got hit in order to stop him resisting and get him in) and they decided to call the EMT and keep him restrained until EMT arrives. Chauvin did not believe him when he said he can't breathe, and didn't want to give in to the bystanders only making things worse.
You are posing the question - did they punch him and not believe him because he was black, or because he was a big guy resisting arrest? We can't answer that. Maybe they would have used the exact same procedure on a White guy. But what we do know is that police completely failed in assessing the situation, Floyd's status (mental AND physical), used excessive force inside the car and outside (and also right from the beginning, where Kueng was pulling a gun on Floyd while he was in his own car and put handcuffs on after) and severely contributed to his death.
I do think Chauvin is at best average IQ, probably low IQ (as are most police officers) and was working like a machine doing what he was trained and read in the books (neck restraint taught by Israeli police/military, keeping target immobilized until EMT arrives), and further incinerated by the crowd telling him he is about to kill someone (I know what I'm doing, don't tell me what to do!).
Racism could be a factor, I don't think it is, but regardless NO TEAM OF POLICE OFFICERS SHOULD EVER REACT LIKE THAT IN THIS SITUATION, no matter who the subject is. If it's not racism-based and they would have done exactly the same to a white 6'6" 230 lbs guy resisting arrest, it is STILL BAD/UNACCEPTABLE. The question is - how can we make police officer a more valued job and get better people for it AND afford better training? By putting more tax money into the police, allowing for longer training and higher salaries. Higher taxes would be needed for that.