Or are there some less known niches to do good, where my money or personal projects would have an even larger impact?
(Please do not discuss here the tradeoffs between helping people suffering now or preventing that people suffer in the (far) future. I want to focus on short term altruism)
After that, consider that very large charities might have considerable overheard and their practical aid on the ground very limited, or very politicized.
Local community organizations might have higher needs, have and allow you to make more more impact, though they will not have the marketing and outreach capabilities of a large organization. But...finding those might take a little more work on your side. As you want to do good in the short term, and I assume you wouldn't mind a local focus and getting the feeling you're contributing something, you could get in touch with e.g. a city social services department, they probably know which organizations exist close to you which could use some help.
Oh also... Work really really hard at being nice person to everybody you meet. From the janitor to the CEO.
Anybody can donate money.
Stop worrying about "the most efficient" everything
Just go DO it - what's "efficient" (whatever that means) for you may or may not be "efficient (again, whatever that means) for me
Just do whatever good you can do. If you're in a position where a high-paying job in the west is something you're likely to slip into anyway and none of the other roles you'd be relying on seem appealing or likely, then sure, donate some of that high salary. But we can't possibly recommend unqualified that everyone should do that.
The idea of earning money doing X and giving it away seems popular, and most startups claim they are "going to change the world" (presumably for the better) but you don't see as much about creating/running businesses that have a positive impact on society and/or the planet.
Do people just not believe that is even possible? Or is there too much risk of failure?
I think most folks will still want something to do - and a purpose - post-retirement, especially if it's early. It would be nice to live to work, rather than work to live...
Good luck with your target!
I believe that knowing how the world actually functions is best for avoiding harm in both the long and short term.
There're [nominally] 8 billion people on the planet
A billion bucks is only twelve cents per person