I personally don't think the risk is currently high enough to justify evacuation if you live in the US (I'm not sure where you're writing from). I think looking at escalations/de-escalations of conflict between nuclear powers as signals of risk makes sense. You could look at estimates like this one (https://forum.effectivealtruism.org/posts/KRFXjCqqfGQAYirm5/samotsvety-nuclear-risk-forecasts-march-2022) or check relevant metaculus questions.
Yes, if imminent literally means missiles are inbound it is too late, but if you've decided there is a high probability of nuclear attack in the next couple of weeks to months evacuating could still be a good strategy. For Ben Landau Taylors signup list, he certainly means evacuating well before missiles are launched.
Certainly small towns are not at much risk of being hit directly. If you were concerned about an all out war between the US and Russia though, evacuating to somewhere in the southern hemisphere could make a lot of sense.
Yields of nuclear weapons can vary a lot. Like you said, no one knows exactly where would be targeted, but if your near a large city that might be hit, considering how to shelter and evacuate after an attack could still be quite useful. I agree attempting to evacuate a city as missiles were being launched would not result in good outcomes.
Yeah, I agree there's a bunch of bio researchers who are fine talking openly about scary stuff you could do with bio and sometimes fiction authors represent that. I think the effect this should have on those that care about infohazards is to be willing to discuss them in order to get work done to prevent them or aid us in the case that they happen. It's hard to justify preparing for something if you're totally unwilling to acknowledge the things you want to prepare for or prevent.
To avoid as much of the fallout as possible. I suppose evacuating opposite of downwind would also be good, but less important to spell out since to evacuate opposite of downwind you would have to already be opposite of downwind. Basically don't go closer to the blast or closer to downwind of the blast if at all possible.