Many mathematically talented people I know (those who perform well in the International Mathematical Olympiad (IMO) or the Putnam competition) go into derivatives trading and finance simply because a) it's mathematically challenging b) there are answers and rewards for getting things "right" (you get more money) c) it's competitive (some of these are zero-sum games played against other intelligent market actors). I am curious as to fields they could join that would mimic this environment, but be highly impactful?
As a person from a similar background (IMO medalist, etc.) who has considered similar criteria, one field I have been thinking a lot about is formal verification. It's mathematically challenging in that it can involve a lot of complicated, unintuitive mathematical logic, and it's verifiable in that you can know when the proof checker succeeds that you got things "right", although I don't think there are rewards like in trading or ML competitions, nor is it competitive in the way you describe. (Personally, I now think this last point is a good thing—I loved the math but hated the zero-sum nature of math competitions. But, if this counts for anything, I think there are people trying to build an AI to "win an IMO gold medal" with formal proofs.)
I am not very confident about the impact of such a career, but I think there's at least a decent chance of high impact. 80000 Hours has a short review with some reasons to believe in its potential and some cases for being bearish, although the latter cases focus on AI safety applications and I think applications to cybersecurity are more direct and also have solid potential for impact. Also, I suspect that a formal verification career that's optimized for impact will look completely different from one that's optimized to be mathematically challenging. But I would guess that, at the very least, the path to high impact is more plausible than derivatives trading or finance.