As mentioned in the title, when you look back on your master thesis- the one that brought you to your Phd life, which advice that you think if you had known them before, you would have had a better thesis and a more promising academic career?
I think those advice would vary from advice on relationship with advisors, work-life balance to something more specific like "Do not get carried away too much by the premature results that you come up with" -which may not be true for everyone.
I expect to learn the most from you, more about your failure and how you cope with them than your success.
Sincerely,
