In addition to Alex's response, I think it could be worth learning a specific domain. For example, I have a large number of friends starting climatetech SaaS/fintech companies- there's a fair amount of domain expertise to learn in addition to everything Alex mentioned.

@Alex what do you think of the thesis of https://www.breakoutlist.com/why-join-a-breakout , which says: high company employee growth /fast growing company -> lots of problems to be solved -> more opportunities to solve them -> faster career growth? It has a list of series B/A companies (Airtable was on the list when I joined Airtable a few years ago) that fit the criteria