As per usual, you make a compelling case, based upon reality.
I'm trying to square it up with my knowledge of our system and what goes on here. One thing I do note is that if an event similar to the Floyd case - ie one of the same level of obvious and deliberate brutality - happened here, one of the first things to occur would be calls for the resignation or dismissal of the Chief Constable of the force concerned and for a full Public Enquiry. Secondly, investigation of the event would be undertaken by officers from a different force. I have not heard of any demands for the resignation or dismissal of Chief Arradondo, have any such occurred? I am unsure of what the US equivalent of a Public Enquiry would be, but is something of this nature likely to follow? I understand that investigation of the case was undertaken or supported by the FBI, an organisation for which we have no equivalent.
It seems, then, that here in the UK there is a greater tendency for the public to place blame and demand accountability at the hghest, rather than the lowest, level. A class-based assumption which holds that the 'woodentops' (uniformed constables), are working class and ipso facto incapable of doing anything right unless kept under control. Thus any bad outcome is seen as a failure of control and the fault of those in charge.
Deaths do occur as a result of police action here. Shootings are rare, of course, as the use of guns even by criminals is less common here, and the deployment of an Armed Response Unit is deemed a last resort. Other deaths have been associated with: improper restraint (chokeholds and headlocks); the use of spit-hoods, Tasers and pepper sprays; RTAs during pursuit. In many such cases 'failure of oversight' is part of the conclusion after investigation, so we do have some of the same problems! Though there is more of a tendency to hold supervisors responsible for failure to enforce regulations.
In the matter of accountability, all police ranks in a force are ultimately accountable to the Chief Constable, of course. The Chief Constable is accountable to the Police and Crime Commissioner for the force, or the Mayor if the force is contained within a metropolitan area. These elected officials are of course accountable to the public.
Over and above this is HMICFRS (Her Majesty's Inspectorate of Constabulary and Fire & Rescue Services), a body which has the stautory responsibility of inspection of all forces, and is accountable to Parliament. Is there any such regulatory body in the US? Is indeed the creation of such a body possible within your legal framework?
What is clear is that you are correct in your insistence that focus must be shifted from individual officers' misdeeds onto the systems that permit them to occur. Which means a cutlure change not only for the police, but for the public and the media.