Yeah, saying millionaire/billionaire as a category is gibberish. You have to be a millionaire if you want to do something crazy, like, I dunno, retire comfortably when you're 65. You have to be a billionaire, well, never. Nobody needs to be a billionaire.
I'd like to be a millionaire, but that is an method to achieve another goal, not the goal itself.
I'd like to vertically farm algae on an industrial scale as an atmospheric carbon sink, and additionally see if there is any way to do so profitably while remaining carbon negative. 1 million dollars would probably be enough to construct a small facility and hire the staff and experts I would need to figure this out.
"Millionare/billionaire" as an identity should be an insult.
Anybody with an industrial mindset will bare minimum want to take any large amount of liquid asset and turn it into production. Build and improve factories, fund research, improve living conditions near your operations. Having money acruing interest can be part of that, but should not be your primary revenue stream.
Don't wait to be a millionaire for that. See if you can find people who are also interested in it and build it as a co-op.
That said, I think you'll run into problems with the square-cube problem. Volume of algae goes up by a cube factor, but they need light. That light has to be fed by energy that has to be gathered by taking up surface area of the Earth, like solar and wind, but that only goes up by a square factor. Hydro and nuclear sources are maybes here, but you'd have to scale way, way up to make that viable (many billions of dollars, not millions). Presumably, you don't want to use energy from carbon-based sources just to suck that carbon back into algae.
Being a millionaire is a good goal if you want to be able to retire before you're ancient. You need something like that just to live. No one needs to be a billionaire - the difference between a million dollars and a billion dollars is staggering.
Nobody wants to enter professions that require hard work because it pays pittance relative to popular entertainers, and traditional milestone purchases have become moving targets, so why not gamble it all?