I've seen people take the advice of "build something that could be useful to someone" the wrong way so many times.
That "someone" shouldn't be an abstract, hypothetical customer but a group of people that contains someone you intimately know, such as a close friend, partner or yourself.
I think "build something that would be useful to someone that you personally know, and many others like them" is a much more concrete way to think about the problem, and will enable you to achieve success with high probability.
(Just as a disclaimer, I started earning enough income from side projects for both my wife and myself to quit our full-time jobs about a year ago and start a family, but we're nowhere near the multi-millions in yearly profit that OP or bootstrapper10x are.)
This is excellent advice. You should know and understand who your customer are. Otherwise you won't even know what to build. Or you will build the wrong product and won't get any feedback.
Congrats on your journey and best of luck for the future!
(Just as a disclaimer, I started earning enough income from side projects for both my wife and myself to quit our full-time jobs about a year ago and start a family, but we're nowhere near the multi-millions in yearly profit that OP or bootstrapper10x are.)