"Want to get to point X? Visit point Y first!"
Just go to point X directly. Too many people shy away from their dreams and delay, because they want to learn first, practice first, and get ready before the real adventure. You will never deplete "more things to learn". You are only depleting your lifetime. And no matter how close working at a startup is to working on a startup, it will never be as close to it as itself.
If you need to save up some money or take care of any other blockers first for your own safety, fine. If there is a great company working on a great problem that you are excited by, great. But if you want to start a new company, then working at a startup is not a practice for starting a new startup. It is a practice for working at a startup and a practice for wasting another day of your life not living the way you really want to live.