What advice would you give to a new technical co-founder at a startup?
My answer to What advice would you give to a new technical co-founder at a startup? on Quora
It sounds to me like you’d be better off seeking an engineering role in an existing, relatively mature startup (one with 5-15 engineers) rather than looking to be a technical co-founder of a brand new company. Startup engineering is likely to be pretty different from what you’re used to, and there’s no better environment to learn than a fast-moving company that already has a small and highly talented group of engineers.
Make sure you pick a company with a great engineering team though!