I actually agree with a lot here. If the question had been "how can I have a fulfilling hobby while also working full time", i think I'd agree with a lot of the advice you gave. It's very good advice about time boxing, having realistic goals, and looking for a well rounded life. Having hobbies is something that can be really rewarding, and hobbies can be many kinds of things, from deeply technical to purely relational and social.
However, I will admit my bias here, the OP explicitly talked about "a startup", and I don't believe I know of too many hobbies that allowed someone to replace their income and retain their standard of living. The people I know who have done this have, regularly, done so with significant risk, commitment, and single-mindedness of purpose. I'm sure there are exceptions as those you listed, but I personally haven't experienced them.
However, I will admit my bias here, the OP explicitly talked about "a startup", and I don't believe I know of too many hobbies that allowed someone to replace their income and retain their standard of living. The people I know who have done this have, regularly, done so with significant risk, commitment, and single-mindedness of purpose. I'm sure there are exceptions as those you listed, but I personally haven't experienced them.