Some relevant questions that come to my mind:

• To what extent do you want to be happy? Have a big impact? Pursue other goals? (The reason I ask is because I see trade-offs)
• If you really value having an impact, what are the fields you could enter where you'd be impactful? Neuroscience? Anti-aging? Cryonics? AI?
• How important do you think each field is? How much value do you think you'd add? How long would it take for you to be productive (although I don't think that's too important a question, assuming you're optimizing for the long-run)?
• How big of an impact do you think you could have in academia? Will you be slowed by bureaucracy? How happy will you be?
• Taking a step back, what are some alternatives to academia that would enable you to have a big impact? Earning to give is a popular option, but what else is there? Politics? Startups? Working on things that augment our ability to think? What sort of schemes could you think up?

My overall impression is that you seem like the kind of person whos happiness is largely tied to how big an impact you have, and who is rather altruistic. And so it seems important for you to find something that enables you to have a big impact. You also seem young enough where it seems worth thinking about the long-run. It seems worth taking the time to do a lot of research and make the right decision.

Most Americans spend too little time on higher-level actions, like being strategic - doing a quick analysis of what your goals are, and which Level 1 or Level 2 actions would best accomplish those goals. Witness the hordes of lawyers who spend thirty years on the Level 1 action of working at a law firm, three years on the Level 2 action of getting a law degree, and three minutes on the Level 3 action of deciding what to do after college.

• Levels of Action