The Policy Impacts-project at Harvard University might be of interest to you, including their method for evaluating the impacts of public policies: the Marginal Value of Public Funds (MVPF).
The MVPF-method measures the “bang for the buck” of public spending on a given policy. How? It’s calculated as the ratio of two numbers: the benefits that a policy provides to its recipients (measured as “willingness to pay”), divided by the policy’s net cost to the government (including all long-term effects on its budget).
The Policy Impacts-project are also collaboratively building a Policy Impacts Library, a database of MVPF-estimates for different public policies derived from rigorous empirical research. The goal is to help policymakers and practitioners better understand and compare the long-term costs and benefits of a wide range of policies.
PS. I find this project super interesting, but have not looked into it in detail nor talked to anyone working on it. So there might be obvious weaknesses I’m not aware of.
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):
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.)