"On the other hand, almost every successful founder I spoke to, either did not have this issue (founded right after school, etc.), or did not do any work before

"On the other hand, almost every successful founder I spoke to, either did not have this issue (founded right after school, etc.), or did not do any work before quitting (which they did either before or after securing funding)." That's the "Burn the Ships" idea, but there are plenty of start-ups that were side projects. I don't think big companies care about your start-up idea at all so don't worry about it too much as long as it's not in direct competition with your employer.