It's actually an awful idea. The whole benefit of working for a large company is that you DON'T need to sacrifice outside of your 9-5. This is that outdated grind mentality that managers sell to convince people this is needed to get ahead before they hand out promotions to their mates anyway.

Secondly, and I don't mean to disrespect you in any way, but if you're self-taught, and your prior experience is one man jobs working on small sites then I wouldn't expect the quality of your code to be great. I would be wondering how much time it would take to ensure whatever you build is really readable, documented, fully tested, maintainable, in line with the company's current tech stack, performant and scalable.

The benefits of working at a large company is that they have products that run on massive code bases that are, for the most part, REALLY STABLE. They move slowly, but they also move surely. New code won't be scrappy and volatile. Old code can stagnate, but it's because it hasn't broken in years. All of these things are learning opportunities you can take with you even if you strike out to start/work at a startup.