That said, for non-civil servants—I would strongly suggest focussing the first role you get on skill development and understanding how government works, rather than trying to go straight into the most impactful area.
Civil servants are expected to move around a lot, so you can find the most impactul areas once you are in, and demonstrating good generalist skills is one the key things that will make moving around easy.
These 2 apps should help give a sense of where you can work on different causes in the Civil Service:
https://highimpact.shinyapps.io/impact_areas/
https://highimpact.shinyapps.io/civil_service_jobs_explorer/
That said, for non-civil servants—I would strongly suggest focussing the first role you get on skill development and understanding how government works, rather than trying to go straight into the most impactful area.
Civil servants are expected to move around a lot, so you can find the most impactul areas once you are in, and demonstrating good generalist skills is one the key things that will make moving around easy.