Having separate taser and firearms causes it's own batch of problems. For instance the 12 yo that was just killed. Cops were responding to a person with a gun call, they had to have their firearms out as there is no time to switch weapons while someone is pulling a gun. There are way too many reports of people not going down immediately when tased, it cannot be relied upon in life or death situations.
Maybe the correct answer is when responding to a gun call, responding officers should have a long gun with less lethal attached (ie. bean bag gun where grenade launcher would mount, is this even possible?). That allows cop to select between lethal and less lethal is substantially less than a second. If they don't want to buy new equipment, load shotgun with a beanbag round in the chamber and lethal shells for the rest, if they don't go down on the first shot, it's game over for them. If it's an active shooter that demands immediate lethality, just pump the beanbag round out before engaging.