I think it's desirable that you have the agency investigating itself to improve itself in the first instance, and that means police investigating police. If you then have the commission as the fallback to carry out the complaints that are not adequately reviewed, I think that is going beyond.
There's then the question of whether you want the commission to hire current or former police officers. That's an administrative decision. I would leave it to the commission to decide.
It obviously then raises the question of whether this commission is independent enough. I think the answer is it will be, but it will have to pay attention to whom it hires.