I started working in startups two years ago at 18. I've never seen any formal training anywhere I've been which makes sense as startups just don't have the time

I started working in startups two years ago at 18. I've never seen any formal training anywhere I've been which makes sense as startups just don't have the time and resources to do it. I've heard that banks are very willing to hire graduates as interns even without any programming experience and train them up from people that went that route, but from everything else I've heard you'll be better off elsewhere in every respect. As a junior developer elsewhere you'd hopefully be surrounded by people that can mentor you and you should learn a lot anyway. As you said I'd build up an OSS portfolio if possible, working on side projects and such when possible as a way to learn and experiment as well as have something to show employers.