Before one can deduce anything from the action axiom, one must state its meaning. As human beings we understand the meaning of human action from reflecting on our own experience in taking actions. We don’t deduce the meaning of uncertainty from the action axiom, we understand what uncertainty means by reflecting on the actions we have taken. From that knowledge we can deduce the laws of uncertainty. In other words, we have to explain the meaning of your step (1) before we formally deduce other propositions from it. We gain this meaning by reflection.
Take a look at David Gordon’s discussion of action in chapters 2 and 3 in his book: