More effective would probably to buy yourself (or a person that has political potential/influence with the right intentions) into politics like president muskrat, and do well placed populistic propaganda against the actually evil fossil industry. You don't even have to lie, just do some good rethoric speech, ads etc.. We need policies on a larger level, and I think 10B$ should be enough to gain significant political influence for something that the major mass of people is already behind of, just needs a good spark. I think Thunberg has shown that. Not that we shouldn't of course improve public transport in cities. Additionally do (employ) investigative journalism probably in the same process to give all of this a good foundation..
Ya its a decent start. My point here is the scale of what we're facing. Its going to take trillions of dollars, synchronous action taken by many nations simultaneously and a complete restructuring of the current world order. There's always hope but at this point i'm fairly cynical and don't have a lot of faith that the right people are in charge to even start the conversation.
I think they're just asking, from a categorical imperative perspective, what is the most effective way for all individuals with a bit of savings to help the climate situation.
And tbh, at this point OP is probably better off spending that $10k on preparing for:
inclimate weather (HVAC, water proofing, warm clothing)
inconsistent power (battery backups, a generator)
food/clean water shortages (home gardening skills, rain catching/water purification).
At this point, there's virtually nothing that can be done to stop the impact of climate change, there is only adapting to survive it. The best we can do is vote and/or hope for our global political situation to finally reach its inevitable crisis point. But I don't expect that to be a pleasant experience.