On the assumption that you are arguing in good faith (which is a core guideline of this site)...
> You've got a split second to decide whether the suspect is reaching for a gun or not.
What are your goal posts for this excuse?
For example: what would an officer have to do to you or your family to make you turn on the officer's actions and demand accountability? It's important to remember that officers frequently demand to see something from your wallet or phone (both of which are located around your belt, where you might have a gun stored).
Also ironic is that police are completely capable of restraining themselves when other officers have guns and when white men carry guns into a state capitol. It's not simply the presence of a gun that police are reacting to.
> make a mistake another way and we'll throw you in jail for two decades.
This is literally the law for civilians. I don't see how you can expect an officer to enforce a law that they, themselves, are not willing to be subject to.
Personally, I think the only question is how much good faith should we give officers in questionable cases of use of force.
> Who in their right mind would take that kind of a deal?
I think every public school teacher in America asks themselves that question every week, but it's still one of the most common professions. People weigh their personal values against the value of their job.
> And if nobody in their right mind would do, who's going to keep the tough guys from imposing their own rule at gun point?
Logical fallacy. The people that are currently officers wouldn't suddenly vanish from the face of the Earth. We would still have private security and parents and teachers and neighbors and business owners and good samaritans.
Police officers are generally very strong proponents of the 2A, whose lobbies love to repeat the mantra "When seconds count, the cops are just minutes away". Without police, the average person would step up and defend themselves more than they do now. Game theory.
I think the biggest problem with US policing in the US is that there are always people who post what you did. It's not a balanced evaluation of what would actually happen if there were more reasonable accountability on police actions but rather "do whatever police unions want because otherwise morale will fall and chaos will reign". Maybe you should consider the morale of the people who see police in their neighborhoods and houses as the same people who are "imposing their own rule at gun point".