First document every "near-miss" or risk you find.

Go make the exact same argument to your CEO, $XX million start-up flatlines with a single slip up.

Make it as easy as possible for your CEO to go to the company board, repeat the argument, and make an ask to shareholders for a loan to hire a security expert. Your self taught experience will help in finding the right person.

Befriend the new security person and learn everything they have to share.