Frame challenge: You cannot use the "start of an attack" as a valid trigger for Ready.
Ready in the Gamemastery Guide has this important line to aid in adjudicating valid triggers for Ready:
Notably, the trigger must be something that happens in the game world and is observable by the character rather than a rules concept that doesn’t exist in world
In order to reach this situation, you have to Step (or use some other movement) in response to an enemy starting to attack you, but that's only a definable point in game-logic. Take, for instance, this video on how to do a roundhouse kick. The first step is to, well, step. So does every step count at the start of a roundhouse kick? What if the person is just stepping into a more aggressive position? It's not actually clear when an attack starts, especially if one wants to differentiate between an attack and a feint.
It's further clear that we are already not reacting to game world events by the shear amount of meta-game confusion this stunt causes, as demonstrated by your question. Even the video you linked expresses a fair amount of uncertainty at whether this stunt actually works, shown by the many hedged statements and their repeated insistence at "needing to see what happens in the narrative".
Yes, this is technically in the realm of GM fiat, and perhaps your GM will declare it as a valid trigger. But at that point, you just do what the GM says, and there is no RAW (nor even RAI, I would argue) guidance left. Option 3 seems the only one that doesn't result in rampant abuse of this stunt, if that is the case, and what I would recommend as a (house) rule to any GM who wishes to allow the stunt.
Note: there are many reactions in the game which have mechanical triggers. Ready simply isn't allowed to use those, and must be described in non-game language.
Note 2: Nimble Dodge already fills the narrative space for this kind of action (dodging out of the way of an attack at the last minute). You may also be interested in Reactive Shield if you are looking for reactions of this nature.