On the topic of policy work in smaller countries, Founders Pledge write the following in their article about Longtermist Institutional Reform (with my emphasis added in bold):

One potential avenue to encouraging global solutions without acting directly on global institutions is to consider that some small countries can have outsized influence as role models or policy exporters. If it is easier to implement long term-focused policy in these smaller jurisdictions—and there is some evidence that this may be the case—doing so may be of large marginal value.

Also, have a look at this blog post: Why scale is overrated: The case for increasing EA policy efforts in smaller countries - EA Forum (effectivealtruism.org). 

If accepting the assumptions above, another reason to work on policy change in smaller countries (as well) is to duplicate efforts. Significant policy change is often dependent on policy windows and luck, and the more efforts in parallell, the bigger chance of success in at least one country. Also, the risk of failure (and potentially politicizing the issue for good) is smaller if in a small country. After several parallell attempts, policy advocates in the US (or other large countries) can refer to the successful example in whatever smaller country where the campaign was successful (NZ, Australia, the Nordic countries, etc.)

This is super interesting! The Policy Impacts-project at Harvard University, and their method for evaluating the impacts of public policies that you refer to, the Marginal Value of Public Funds (MVPF), is new to me but seems potentially very valuable! 

I recommend any policy-interested EA to check out the links I have provided here (very simple reading), and those from @Schethik above. 

I see that the Policy Impacts project have also created a Policy Impacts Library, a database of MVPF estimates for different public policies derived from rigorous empirical research. 

I fully agree with this!

"it doesn’t feel like the EA community has thought much about policy. For example there is a huge focus on AI policy, but the justification for this is weak. Even if you fully believe the longtermist arguments that top programmers should work on AI alignment, it does not immediately follow that good policy people can have more long term impact in AI policy compared to policy on resilience, macroeconomics, institution design, nuclear non-proliferation, climate change, democracy promotion, political polarisation, etc, etc."

EA Australia does this as well (for many years), and donations through them are tax deductible. See https://effectivealtruism.org.au/