I think the key to improving police treatment of arrestees is an improvement in recruiting of police. The bully wannabees must be screened out before they are hired. I don’t know how you do that, but it needs to be done.
Another other key problem is blue omerta. When cops know that other cops won’t be witnesses against them, it emboldens the abusers to be abusive. The growing ubiquity of cellphone videos helps.
Body cameras will go a long way to preventing unwarranted beatings. The cameras should be on all the time, and not under the control of the wearers. Something will have to be done to protect police officers’ privacy during bathroom breaks, but pointing the cameras at the ceiling would solve that problem. edited But it can be worked out.