You could try doing something relevant to one of the charities founded under Charity Entrepreneurship. They often advocate or implement practical and widely-recommended policy solutions within specific low-income countries. There’s one charity working on reducing lead exposure for children, and another trying to improve vaccination rates against preventable diseases. You could reach out to the founders, learn which countries they’re considering working in next, write a general profile of poverty with specific focus on their target problem, advocate their policy or analyze their options within that country, and provide them a stakeholder roadmap for getting the policy effectively implemented. Charity Entrepreneurship also has a big list of ideas they’re waiting for more people to tackle, so you could talk with CE about fleshing out some of those proposals.

https://www.charityentrepreneurship.com/our-charities

I have a Gdoc here ('Appendices to "The case against randomista development"') where I rank countries as a function of population, poverty (GDP per capita) to get at the importance and neglectedness of improving their economic policy proxied by their ranking on the World Banks Doing Business ranking.  The ranking is below.

But this didn't include tractability. Also from a longtermist perspective some people say that India is probably going to be the most important country, due to its population size, democracy, scalability, and so on.