It takes a lot of people to launch a nuke. While missile operators are trained to act quickly, they are also drilled hard on adherence to policy. A 94% on the test for that policy is a failing grade.
And while I think you’re very right to not trust the US or Russia to adhere to treaties, if said treaty requires that training policies and doctrine reflect the no first strike stance, that would mean a whole lot of people would have to be willing to violate that treaty in order to launch first. Heck, there’s been incidents during the Cold War where a single person’s hesitancy to follow approved launch policy has averted total nuclear war.
I think a treaty and accompanying training and doctrine could create sufficient barriers to make a nuclear first strike far less likely, though, of course, not impossible. But that alone seems like a worthwhile thing to pursue.