Although working with the police detective can be helpful, I hope you are confident that you are not being investigated for possible charges at the same time.
I recommend you make appointments with lawyers and meet with a few before saying too much to the detective. This is to better ensure your rights are being protected.
Take your medical bills with you when you meet with these lawyers (if you have them). If you have a copy of the video that shows your unfortunate encounter with the officer, take that, too - although the attorney can gain access to it. If your witnesses have already written out their statements in police complaints, those can be important should this wind up in court. You might want to ask the witnesses if they can be available in the future to testify.
You can discuss various legal options with the attorneys you see.
You should ask about attorney fees prior to setting up any meeting. Most attorneys will offer some free minutes of their time to review the facts (often 15-30 minutes) but you might have to pay for an hour or so for their review.
Good luck.