I meant something like Washington DC's OPC[1]. Note that these commissions are not themselves perfect; they're typically limited in authority to "recommendations," which the police (or DA) may still ignore.
However, to answer the following (which isn't what I meant, but I think is interesting):
> but assuming it’s a private firm would you not have the same criticism that it’s not a neutral third party and is biased towards whoever hired them?
I think the answer to this is similar to the answer found in arbitration: arbitration is typically performed by a well-known firm, and that firm is obligated through contract to perform fair proceedings. In other words: there are civil penalties on the table if the independent party does not execute their job fairly.
That being said, I think an unrelated but still governmental investigation arm would be sufficient here.
> What is the role of a state level investigation bureau if not to investigate state-level issues involving state agencies?
You can find the GBI's services listed here[2]. They have a lot of responsibilities, most of which aren't tied to investigation of local police. I think it'd even be fair to say that they aren't particularly equipped to investigate local forces.
[1]: https://policecomplaints.dc.gov/
[2]: https://gbi.georgia.gov/services