Just a thought: You can generate $15k of monthly passive income directly with $300-350k investment. If the bootstrapped business fails to make money fast, there's always this option with your amount of money.
Developers often miss those obvious ways to make money because these ways aren't technically intricate.
Source: I do affiliate marketing. This are the usual ROIs.
Good luck on your journey!
edit: wrt. the downvotes - that's one of the reasons I don't want to talk about this in public. I agree that it sounds unbelievable, but most people also don't believe in $500k salaries. Many people dismiss those opportunities because they don't believe it's possible.