Reposting an anonymous addition from someone who works in policy:
Your list of options mostly matches how I think about this. I would add:
Based on several anecdotal examples, the main paths I’m aware of for becoming a trusted technical advisor are “start with a relevant job like a policy fellowship, a job doing technical research that informs policy, or a non-policy technical job, and gradually earn a reputation for being a helpful expert.” To earn that reputation, some things you can do are: become one of the people who knows most about some niche but important area (anecdotally, “just” a few years of learning can be sufficient for someone to become a top expert in areas such as compute governance or high-skill immigration policy, since these are areas where no one has decades or experience — though there are also generalists who serve as trusted technical advisors); taking opportunities that come your way to advise policymakers (such opportunities can be common once you have your first policy job, or if you can draw on a strong network while doing primarily non-policy technical work); and generally being nice and respecting confidentiality. You don’t need to be a US citizen for doing this in the US context.
In addition to GovAI, other orgs where people can do technical research for AI policy include:
RAND and Epoch AI
Academia (e.g. I think the AI policy paper “What does it take to catch a Chinchilla?” was written as part of the author’s PhD work)
Reposting an anonymous addition from someone who works in policy: