Where are these documents, and what do they verify?
Saying that an institution today goes back to an institution from the past doesn’t mean that it shares the same traits. Institutions change over time.
What you need to show is that the same purpose can be documented today as was there when the police were established.
From what I've read (e.g. https://plsonline.eku.edu/insidelook/history-policing-united...), the "slave patrol" thing is only part of the story. If anything, policing was developed first in the 17th century in England and the 13 colonies, and the slave patrols evolved to match the modernizing template of a police force.
If you want to lean a bit farther left, the obvious implication here is that police exist to benefit those who have power and property, projecting the former and protecting the latter; if necessary, abusing those who lack power/property in the process.
That policing ends up being "racist" then is then a natural consequence, and not at all an axiom.
Law and "order" can also mean keeping the order of classes as is.
The best people I would ever meet would not protect bad cops at every opportunity. The fact that police who stand up to bad policing are consistently removed from the ranks does not lend credence to your assertion.
https://www.theatlantic.com/ideas/archive/2020/07/what-polic...
https://en.m.wikipedia.org/wiki/Warren_v._District_of_Columb....
If you read about the historical origins of the US police force, their (verifiably documented) lineage goes back to the days of English colonialism. Those original patrol groups were created to "protect" the damage/loss of property- namely to prevent runaway slaves from escaping their owner's control.
The US police force has, quite literally, always been first and foremost a way for the richest upper class to protect their wealth, power, and assets. They are a way to keep the status quo. The fact that they (sometimes) help poor and working class citizens is merely a by-product of their primary goal.